Fax API: How to Send Faxes Programmatically (2026)

By Alexey Spasskiy · Published March 22, 2026 · Updated June 8, 2026 · 14 min read

Quick Answer: A fax API lets you send and receive faxes through simple REST calls — no fax machine, no phone line, no protocol knowledge. Upload a document, provide a phone number, and your code handles the rest. The mFax API sends a fax with a single multipart POST /v1/faxes request, and mFax Business includes API access with HIPAA compliance starting at about $9/mo (billed annually).

Fax is still deeply embedded in healthcare, legal, finance, and government workflows. Over 9 billion fax pages are exchanged annually in US healthcare alone, and 89% of organizations continue using fax for critical document transmission. But nobody wants to manage physical fax machines — especially developers building modern software.

A fax API bridges the gap. It lets your application send and receive faxes programmatically through standard HTTP requests, while the provider handles the analog telephony, protocol negotiation, and delivery confirmation behind the scenes.

This guide covers how fax APIs work, what features to evaluate, how leading providers compare on pricing, and how to integrate fax into your application step by step — with working examples from the first-party mFax API (built by Octagon Lab) and honest comparisons to the other major providers.

What Is a Fax API?

A fax API is a set of REST endpoints that lets software send, receive, and manage faxes programmatically. Instead of feeding paper into a machine connected to a phone line, your application makes an HTTP POST request with a document attachment and a recipient phone number. The provider's cloud infrastructure converts the document, establishes a telephone connection, and transmits it as a fax.

How it differs from online fax services: A web-based fax service gives people a dashboard to send faxes manually. A fax API gives software the ability to send faxes automatically — triggered by events in your application (a form submission, a CRM action, an EHR workflow).

Core capabilities of a fax API:

• Send faxes — POST a document with a destination number
• Receive faxes — get incoming faxes delivered to a webhook endpoint as PDFs
• Track status — poll or subscribe to delivery confirmations and error notifications
• Manage records — list, fetch, and delete sent and received faxes through the API
• Store and retrieve — download sent and received fax documents from the provider's storage

How a Fax API Works

The lifecycle of a fax sent through an API follows five stages. The pattern is the same across providers; the concrete examples below use the mFax API.

Why Use a Fax API?

If your application needs to send or receive faxes, here's why an API is the right approach:

Fax API Providers Compared (2026)

Choosing a fax API provider comes down to pricing, compliance, SDK support, and reliability. Here's how the major players compare:

mFax

The mFax API is a first-party REST API from Octagon Lab. Authentication is a single Authorization: Bearer zk_live_... header using a business API key created in the mFax dashboard. Sending is one multipart/form-data request to POST /v1/faxes with just to and file — no connection IDs or per-request configuration to wire up. The sender number is your mFax Business virtual fax number, tied to your subscription. Webhooks (fax.delivered, fax.failed, fax.received) are configured once in the dashboard with an optional signing secret. API access, a virtual fax number, HIPAA compliance with a signed BAA, team accounts, and a web dashboard all come with mFax Business from about $9/mo. Pricing is fully customizable — you build your own plan with a live calculator, picking the exact seats and pages your integration needs and paying only for what you use instead of buying a rigid fixed tier. Read the docs and create a key at developers.mfax.to.

Best for: teams that need both a user-facing dashboard and an API — healthcare practices, legal firms, and SMBs that want a single platform for manual and automated faxing.

Telnyx

The developer favorite for raw API flexibility. Telnyx uses a Bearer token, charges around $0.007 per page with no monthly minimums, and provides SDKs for Python, Ruby, and Node.js. It exposes a hosted media_url and requires a connection_id on send, and its webhooks are Ed25519-signed. The catch: no auto-retry on failed sends, so your code needs to handle retries. Telnyx signs a BAA for HIPAA workloads.

Best for: developers building high-volume integrations who want granular control and the lowest per-page cost.

RingCentral

Part of a larger communications platform (voice, SMS, video). The fax API uses OAuth 2.0 / JWT, supports many file formats and 11 SDK languages, and includes a sandbox environment with free test faxes. Plans bundle fax pages with your seat subscription.

Best for: enterprises already using RingCentral for unified communications who want to add fax to the same platform.

Sinch (formerly Phaxio)

Phaxio was one of the earliest developer-focused fax APIs, now part of Sinch. It uses HTTP Basic auth (API_KEY:API_SECRET) and a multipart file upload. At around $0.045/page, it's pricier than Telnyx but offers automatic retries, a configurable no-retention mode, and a free signed BAA.

Best for: startups and small integrations that prioritize simplicity and don't need enterprise features.

Key Features to Evaluate

Not all fax APIs are equal. Here's what matters when choosing a provider:

Integrating a Fax API: Step by Step

Here's the typical integration workflow regardless of which provider you choose:

Quick Start: Send and Poll with the mFax API

The fastest way to see the mFax API work is two curl calls. Send a PDF, then fetch the fax by the uuid you got back:

# 1. Send a fax (multipart: just `to` and `file`)
curl -X POST https://developers.mfax.to/v1/faxes \
  -H "Authorization: Bearer zk_live_your_key" \
  -F "to=+15417543010" \
  -F "[email protected]"

# 2. Poll the fax by its UUID for status updates
curl https://developers.mfax.to/v1/faxes/<uuid> \
  -H "Authorization: Bearer zk_live_your_key"

That's the whole send path: no from, no quality, no per-request callback URL. The hands-on walkthrough with Node, Python, and PHP examples lives in our send fax API tutorial.

Typical API Request Structure

Here's the real mFax send request. It's a multipart/form-data POST with exactly two fields — to (E.164) and file (a PDF):

POST /v1/faxes HTTP/1.1
Host: developers.mfax.to
Authorization: Bearer zk_live_xxxxxxxx
Content-Type: multipart/form-data; boundary=----mfax

------mfax
Content-Disposition: form-data; name="to"

+15417543010
------mfax
Content-Disposition: form-data; name="file"; filename="invoice.pdf"
Content-Type: application/pdf

<binary PDF data>
------mfax--

A successful send returns HTTP 202 Accepted with the Fax object:

{
  "uuid": "a1b2c3d4-5e6f-7081-92a3-b4c5d6e7f809",
  "status": "queued",
  "direction": "outbound",
  "to": "+15417543010",
  "page_count": 0,
  "created_at": "2026-06-05T14:30:00Z"
}

The fax then moves queued → sending → delivered (or failed). You learn the outcome either from the fax.delivered / fax.failed webhook or by polling GET /v1/faxes/{uuid}, which returns the current status, the final page_count, and a presigned media_url to download the PDF. On a failed fax, failure_reason carries the carrier reason (for example, user_busy).

HIPAA Compliance for Fax APIs

If your application handles Protected Health Information (PHI) — patient records, prescriptions, referrals, lab results — your fax API must be HIPAA compliant. This isn't optional. The largest HIPAA fax violation resulted in a $2.5 million fine.

Here's the compliance checklist:

• ✓Signed BAA: The provider must execute a Business Associate Agreement before you transmit any PHI. Marketing that says "HIPAA-capable" or "HIPAA-ready" is not the same as a signed BAA. mFax Business and Telnyx both sign one; Humble Fax does not, so it's off the table for PHI.
• ✓TLS 1.2+ in transit: All API calls and webhook callbacks must use HTTPS with TLS 1.2 or higher. Reject any provider that allows unencrypted connections.
• ✓AES-256 at rest: Fax documents stored on the provider's servers must be encrypted with AES-256. Check how long documents are retained and whether you can configure auto-deletion. See our fax encryption guide for details.
• ✓Audit logging: Every send, receive, view, and download event must be logged with timestamps, user IDs, and IP addresses. These logs must be exportable for compliance audits.
• ✓Access controls: Role-based access so only authorized users can view faxes. Multi-factor authentication on API dashboard access.
• ✓Secure webhooks: Webhook payloads should be signed so your application can verify they genuinely came from the provider. mFax signs with an HMAC-SHA256 over "<timestamp>.<raw_body>" and sends it in the X-Zelda-Timestamp and X-Zelda-Signature headers; verify in constant time and reject timestamps older than 5 minutes.

For the full framework, see our HIPAA fax API guide and our list of HIPAA compliant fax services.

Common Use Cases

Fax APIs power automated workflows across industries where fax remains the required transmission method:

Healthcare (EHR integration) — Automatically fax referrals, prior authorizations, prescriptions, and lab results from your EHR system. Over 70% of healthcare communication still uses fax, and 56% of medical referrals are fax-based. Integrating a fax API eliminates the manual print-scan-fax cycle. See our guide on faxing medical records.

Legal (document filing) — Courts in many jurisdictions require faxed filings. A fax API lets case management software send filings automatically with delivery confirmation that holds up as proof of service.

Finance and insurance — Loan documents, claims, and compliance paperwork are routinely faxed between institutions. API integration eliminates manual handling and creates an auditable paper trail.

Government (IRS and agencies) — The IRS accepts faxed forms for dozens of submissions. A fax API can automate IRS form delivery from tax preparation software.

Enterprise IT — Replace aging fax servers and phone lines with a cloud API. Companies switching from on-premise to cloud fax report 50–70% cost savings on faxing infrastructure.

Fax API vs. Other Fax Methods

For most businesses that fax occasionally, a cloud fax dashboard is sufficient. For applications that need to send faxes as part of an automated workflow, an API is the only option that scales.

Common API Errors and How to Handle Them

Fax transmission can fail for reasons outside your control. Build your integration to handle these gracefully. The mFax API returns a JSON error body — { "code": "...", "message": "..." } — alongside the HTTP status:

Frequently Asked Questions

What programming languages can I use with a fax API?

Any language that can make HTTP requests — which is all of them. Most fax APIs are RESTful, returning JSON over HTTPS. The mFax API has no official SDK packages, but because it's a plain REST API you can call it from Node, Python, PHP, Go, or anything else with an HTTP client. Some providers do ship SDKs (RingCentral offers 11 languages, Telnyx offers Python/Ruby/Node), but they're never required.

Can I send a fax to an international number via API?

Yes. Format the number in E.164 format (e.g., +442071234567 for a UK number) and the provider routes it through international carriers. International per-page rates are typically higher than domestic. See our international faxing guide for country-specific formatting.

How fast is fax transmission via API?

A single page takes approximately 30–60 seconds to transmit, the same as a physical fax machine — the bottleneck is the analog telephone protocol, not your API call. A 10-page document takes 5–10 minutes end-to-end. The API call itself returns in milliseconds: mFax responds 202 Accepted immediately and queues the fax, so you track progress via webhook or GET /v1/faxes/{uuid}. See our fax transmission time guide for details.

Can I receive faxes through an API?

Yes. With mFax, inbound faxes arrive on your Business virtual fax number: the API fires a fax.received webhook and the document is also available via GET /v1/faxes?direction=inbound. The Fax object's media_url is a presigned, time-limited link to the PDF. If webhooks aren't feasible, you can poll the list endpoint instead.

Get Started with a Fax API

A fax API turns a 1960s communication protocol into a modern, programmable service. Whether you're building an EHR integration, automating legal filings, or replacing aging fax servers, the right API provider handles the telephony so you can focus on your application logic.

For developers, the mFax API gets you sending in minutes: create a zk_live_ key, POST /v1/faxes with a to number and a PDF, and track delivery with webhooks. Read the reference and grab a key at developers.mfax.to, then follow our send fax API tutorial for working code and the HIPAA fax API guide for compliant setups.

For business teams that need both a user-facing dashboard and API access, mFax Business includes everything: API access, virtual fax numbers, HIPAA compliance with a signed BAA, team accounts, and a web dashboard — starting at about $9/mo. You build your own plan, scaling seats and pages to fit your workload with no fixed tiers.

For personal or occasional faxing without an API, the mFax app lets you send a fax from your phone in under 2 minutes.

For a broader look at cloud fax services, see our cloud fax guide or our comparison of the best online fax services.