By David Thompson · Published January 2, 2026 · Updated June 8, 2026 · 9 min read
Quick Answer: Fax over IP (FoIP) sends fax documents across internet networks instead of traditional phone lines. The most reliable FoIP method is store-and-forward cloud fax — which is exactly how mFax.to works. Upload a document, enter the number, send. No VoIP configuration needed.
As businesses migrate from traditional phone lines to VoIP and SIP trunking, a critical question surfaces: how do you keep faxing working? Fax over IP (FoIP) is the answer — but the term covers several very different technologies, each with its own reliability profile.
This guide explains what fax over IP actually means, how the three transmission methods compare, and which approach delivers the best results for your situation. We will also share real-world reliability data that most competitor guides leave out.
Already Having FoIP Problems?
If your faxes are failing on a VoIP system right now, see our VoIP fax troubleshooting guide for 6 immediate fixes. This article is the bigger-picture explainer.
What Is Fax over IP (FoIP)?
Fax over IP is the transmission of fax documents over an IP (Internet Protocol) network — the internet, a private WAN, or any packet-switched network — instead of the traditional Public Switched Telephone Network (PSTN).
Traditional fax machines communicate using the T.30 protocol: a sequence of analog audio tones sent over a dedicated, circuit-switched phone line. That dedicated circuit guarantees consistent timing and signal quality. FoIP replaces that dedicated circuit with a shared, packet-switched network where data competes with voice calls, video streams, and everything else.
The core challenge: fax is one of the least tolerant applications on any network. Voice calls survive 5% packet loss with barely a hiccup. Fax fails completely at 1–2% packet loss on most configurations. Every FoIP method is ultimately an attempt to bridge this gap.
Key terms:
- FoIP — Fax over IP, the umbrella term for any fax transmission over an IP network
- VoIP — Voice over IP, designed for voice calls but often asked to carry fax too
- PSTN — the traditional analog phone network (copper lines)
- SIP — Session Initiation Protocol, used to set up VoIP/FoIP calls
- ATA — Analog Telephone Adapter, a device that connects a fax machine to an IP network
How Fax over IP Works
At a high level, FoIP converts analog fax signals into digital data, transmits that data across an IP network, and converts it back to analog on the receiving end. But the details of how that conversion happens determine whether your fax goes through or fails.
The Fax Machine Initiates
The sending fax machine dials the recipient and begins the T.30 handshake — the familiar sequence of beeps and tones. The fax machine does not know (or care) whether it is connected to a phone line or an IP network.
A Gateway Intercepts the Signal
An IP-capable device — an ATA, a fax gateway, a PBX, or a cloud service — intercepts the analog fax signal. What happens next depends on the transmission method (passthrough, relay, or store-and-forward).
Data Travels Over IP
The fax data is packaged into IP packets and sent across the network. Unlike a dedicated phone circuit, these packets share bandwidth with all other traffic and may arrive out of order, late, or not at all.
The Receiving End Converts Back
A gateway on the receiving end converts the IP packets back into T.30 analog signals (for real-time methods) or delivers the document directly as a file (for store-and-forward). The receiving fax machine prints the page.
The critical variable is step 2 — how the gateway handles the fax signal. That is where the three transmission methods diverge.
Three FoIP Transmission Methods
Not all fax over IP is created equal. There are three fundamentally different approaches, and the one you use determines your success rate.
G.711 Passthrough (Fax as Audio)
The simplest method. The VoIP system treats the fax signal as uncompressed audio and passes it through the IP network using the G.711 codec (64 kbps PCM). No special fax handling — the network has no idea it is carrying fax data.
Pros: Works with any VoIP system that supports G.711. Zero fax-specific configuration.
Cons: No error correction. Fails at even 1% packet loss. Every network glitch hits the fax directly.
T.38 Fax Relay (Fax-Aware Packets)
The ITU-T standard specifically designed for real-time fax over IP. A T.38 gateway decodes the fax signal, converts it into fax-aware data packets with built-in redundancy, and sends those packets over UDP or TCP. The receiving gateway converts back to T.30 tones.
Pros: Tolerates 10%+ packet loss with full redundancy. Lower bandwidth than G.711. The gold standard for real-time FoIP.
Cons: Both sending and receiving gateways must support T.38. Setup is more complex. SIP re-INVITE failures can break the voice-to-fax switch.
For a deep dive on T.38, see our T.38 protocol explainer.
Store-and-Forward (Cloud Fax)
A fundamentally different approach. Instead of sending fax as a real-time call over IP, the document is uploaded to a cloud service, which queues it and delivers it to the recipient via its own dedicated fax infrastructure. The fax is never a "live call" on your network.
Pros: Highest reliability (96%+). No VoIP configuration. Works from any device. No hardware needed.
Cons: Not instantaneous — delivery takes seconds to minutes rather than real-time. Requires a cloud fax provider.
Head-to-Head Comparison
| Feature | G.711 Passthrough | T.38 Relay | Store-and-Forward (Cloud) |
|---|---|---|---|
| How it works | Fax tones as uncompressed audio | Fax-specific data packets | Document uploaded and queued |
| Success rate | ~80% | 85–92% | 96%+ |
| Packet loss tolerance | Fails at 1% | Tolerates 10%+ | Not applicable (not real-time) |
| Setup complexity | Low (set codec to G.711) | Medium (both ends need T.38) | None (sign up and send) |
| Hardware required | Fax machine + ATA | Fax machine + T.38 gateway | Phone, computer, or tablet |
| Real-time? | Yes | Yes | No (seconds to minutes) |
| Best for | Occasional, non-critical fax | VoIP offices that must use fax machines | Everyone else |
Cloud Fax Wins on Reliability
Store-and-forward cloud fax avoids real-time packet loss issues entirely. mFax connects to the phone network through dedicated fax infrastructure — not your VoIP system — delivering a 98% success rate with confirmation receipts.
FoIP Deployment Models
How you deploy fax over IP depends on your existing infrastructure. Here are the four common models:
| Model | What It Is | Best For | Cost |
|---|---|---|---|
| ATA (Analog Telephone Adapter) | A small 1–2 port device connecting a fax machine to your IP network | Home office, single fax machine | $30–80 one-time |
| On-Premise Fax Gateway | Multi-port device (2–24+ ports) connecting multiple fax lines to IP PBX | Multi-line offices with existing fax hardware | $500–5,000+ |
| On-Premise Fax Server | Software on a server with fax boards, integrating with email and EHR systems | Mid-to-large enterprise, regulated data that must stay on-network | $2,000–20,000+ |
| Cloud Fax Service | No hardware — send/receive via web, app, email, or API | Any size business, remote teams, HIPAA workflows | $10–50/month |
How to Choose
Choose an ATA if you have a single fax machine you need to keep using on a VoIP-only network. Plug the fax machine into the ATA, configure G.711 or T.38, and you are running. It is the cheapest path but offers the lowest reliability.
Choose a fax gateway if your organization has multiple physical fax machines and you are migrating from PSTN to SIP trunking. Gateways handle the analog-to-IP conversion at scale.
Choose a fax server if you need deep integration with internal systems (EHR, document management, email) and regulatory requirements mandate that fax data stays on your own network.
Choose cloud fax if you want the highest reliability with the least effort. No hardware to manage, no protocols to configure, and you can fax from any device. This is the direction the industry is moving — the global cloud fax market is projected to grow from $0.85 billion (2024) to $2.05 billion by 2033.
The PSTN Is Shutting Down
Telecom carriers worldwide are retiring copper phone lines. When your analog line goes away, you will need a FoIP solution regardless. Cloud fax is the path that requires the least ongoing maintenance.
FoIP Reliability: The Real Numbers
Most FoIP guides are vague about success rates. Here is what industry data actually shows:
| Method | Typical Success Rate | Key Limitation |
|---|---|---|
| Traditional analog (PSTN) | ~95% | Requires dedicated phone line |
| G.711 passthrough | ~80% | Fails at 1% packet loss |
| T.38 relay (optimized) | 85–92% | Both ends must support T.38 |
| T.38 relay (unoptimized) | ~80% | Misconfiguration drops rates to G.711 levels |
| Cloud fax (store-and-forward) | 96%+ | Not real-time (seconds to minutes delay) |
The takeaway: even well-configured T.38 rarely matches analog reliability. Cloud fax is the only IP-based method that consistently exceeds analog success rates — because it sidesteps real-time packet transmission entirely.
Best Practices for Reliable FoIP
If you are running fax over your IP network (not using cloud fax), follow these practices to maximize success rates:
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Use T.38 whenever possible. If your VoIP provider supports it and your ATA/PBX supports it, enable T.38 relay. It is significantly more reliable than G.711 passthrough. Verify that T.38 is supported end-to-end — not just on your side.
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Fall back to G.711, never G.729. If T.38 is not available, use the G.711 (PCMU) codec. It preserves the full 64 kbps audio signal. Never use compressed codecs like G.729 — they strip frequencies that fax machines need and will cause failures.
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Enable QoS on your network. Configure Quality of Service rules on your router to prioritize fax/VoIP traffic over other data. Fax is more sensitive to jitter and latency than voice calls.
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Lower the baud rate. Set your fax machine to 9,600 bps (some label this "compatible" or "slow" mode). Lower speeds are more tolerant of network imperfections. If failures persist, try 4,800 bps.
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Disable ECM over VoIP. Error Correction Mode causes constant retransmit requests that overwhelm VoIP connections. Turn it off — the fax may be slightly lower quality, but it is far more likely to complete.
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Disable silence and echo suppression. Both features misidentify fax tones and suppress them. Turn them off on your ATA or PBX for the fax port.
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Dedicate bandwidth for fax. If you fax frequently, consider a dedicated VLAN or bandwidth reservation for fax traffic. Fax should never compete with video conferencing for bandwidth.
For step-by-step VoIP fax settings, see our VoIP fax troubleshooting guide.
Test After Every Change
After adjusting any FoIP setting, send a test fax to verify. Test with a single page first, then a multi-page document at standard and fine resolution.
FoIP vs. Cloud Fax: Which Is Right for You?
The honest answer depends on one question: do you have physical fax machines you must keep using?
If yes: You need real-time FoIP (T.38 or G.711 passthrough) to connect those machines to your IP network. Follow the best practices above and accept that reliability will be lower than analog.
If no: Cloud fax is better in every measurable way — higher reliability, lower complexity, no hardware, and the ability to fax from any device. The shift from analog to digital fax is accelerating, and cloud fax is the destination.
| Factor | Real-Time FoIP (T.38/G.711) | Cloud Fax (mFax) |
|---|---|---|
| Reliability | 80–92% | 98% |
| Setup | ATA/gateway, VoIP config, firewall rules | None — sign up and send |
| Hardware | Fax machine + adapter or gateway | Phone, computer, or tablet |
| Maintenance | Ongoing (monitor settings, update firmware) | None |
| Mobile access | No | Yes |
| Delivery confirmation | Depends on configuration | Automatic |
| Encryption | Varies (SRTP/TLS if configured) | TLS + AES-256 |
| Cost | $30–5,000+ hardware + VoIP plan | Pay-per-fax or subscription |
Start Faxing Without the Complexity
Fax over IP solves a real problem: keeping fax working as the world moves from analog phone lines to IP networks. But the real-time methods (T.38 and G.711) carry inherent reliability limitations — because they are trying to force a technology designed for dedicated circuits through a shared, packet-switched network.
Cloud fax takes the opposite approach. Instead of making fax survive on your VoIP system, it moves fax off your network entirely. The cloud provider maintains dedicated infrastructure that connects directly to the phone network — so jitter, codec compression, and packet loss are someone else's problem.
mFax.to gives you exactly this. Upload a PDF or snap a photo, enter the fax number, and send. 98% delivery rate with confirmation receipts. No fax machine, no ATA, no T.38 configuration.
For business teams, mFax Business adds virtual fax numbers, team accounts, HIPAA compliance features, and a shared dashboard — starting at about $9/mo (billed annually). Pricing is usage-based ($3/seat + $4 per 100 pages), so you build your own plan around the exact seats and pages you need instead of paying for rigid fixed tiers.