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FoIP Explained: Sending Fax Machines Over an IP Network
Analog lines are disappearing, but fax has not. Healthcare intake, legal filings, logistics confirmations, and government forms still require a fax number in many regions. Fax over IP, usually abbreviated FoIP, lets an organization keep its existing fax machines and fax numbers while the underlying transport moves from copper to SIP. The practical challenge is that fax was designed for analog circuits, and IP networks are packet-based, so something must bridge the two.
How Fax over IP Actually Works
A fax machine generates a digital image, then modulates it onto an analog voice-grade carrier using a tone-based protocol. On a legacy PSTN line, that analog carrier travels from machine to machine. Over IP, the fax session still generates audio tones, but those tones must be carried across a packet network that was never designed for them.
Two approaches exist. The first carries the fax tones as audio, compressed into a voice stream. This works but consumes significant bandwidth and is sensitive to packet loss and jitter. The second approach negotiates T.38, a signalling protocol that encapsulates the fax negotiation and image data in dedicated IP packets rather than audio. T.38 is far more robust across packet networks and is the method that should be required from any carrier or gateway that supports FoIP.
| Aspect | Audio-based FoIP | T.38 FoIP |
|---|---|---|
| Transport | Compressed audio stream | Native IP packets |
| Bandwidth per session | Higher | Lower |
| Latency tolerance | Low | High |
| Packet loss sensitivity | High | Low |
| Interoperability | Works with anything | Requires SIP signalling support |
| Recommendation | Legacy fallback | Preferred method |
Where Fax Still Appears in Modern Businesses
- Healthcare: referral intake, insurance forms, laboratory reports, and regulatory submissions
- Legal: court filings and notarisation workflows in some jurisdictions
- Logistics: delivery confirmations and signed bills of lading from carrier partners
- Government and insurance: forms that only accept fax submissions
- Senior care: facilities whose partners have not modernised their intake channels
Two FoIP Architectures
Fax on the SIP trunk
The fax number is delivered as a SIP channel. This requires the carrier or SIP provider to support T.38 on the trunk, and it eliminates the on-site equipment entirely. Where a fax number is already provisioned on the trunk, the simplest migration path is to enable T.38 and point the fax machine at the trunk.
Fax through an on-site VoIP gateway
The fax machine plugs into an FXS port on a gateway such as the Redstone RGW series, and the gateway registers as a SIP endpoint. This is the standard approach for sites converting analog lines to VoIP, because the same gateway port that once carried a voice line now carries fax traffic. It also allows a dedicated fax number independent of the main trunk provisioning.
Practical Problems to Plan For
| Problem | Cause | Mitigation |
|---|---|---|
| Fax fails intermittently | Packet loss or jitter on FoIP path | Enable T.38; apply QoS to voice class |
| Slow page transmission | Compression disabled or codec negotiation mismatch | Pin an efficient codec; verify T.38 negotiation |
| Busy errors at peak | Insufficient concurrent channels | Size channels for simultaneous fax peaks |
| One long call blocks others | Channel exhaustion by queued retries | Alert on channel occupancy; plan headroom |
| Fax works on copper but not VoIP | No T.38 support on trunk | Use an on-site gateway instead of trunk fax |
Migration Checklist
- Inventory every fax machine, fax number, and the department that depends on it
- Confirm whether each number is delivered on the SIP trunk or needs an on-site gateway port
- Verify T.38 support end to end, including any hosted PBX or SBC in the path
- Test each fax machine on the new platform before retiring the analog line
- Check whether confirmation reports or delivery receipts are required for compliance
- Keep one analog fallback line if a business-critical fax cannot tolerate any downtime
Redstone FoIP Approach
Redstone RGW series gateways provide FXS ports specifically suited to fax survivability. Because the gateway sits between the analog device and the SIP core, a site can convert analog lines incrementally, moving voice numbers to VoIP while keeping fax working on the same physical ports. For organisations planning a full analog retirement, this lets fax requirements be resolved without holding the entire migration hostage to one stubborn device.
Redstone provides migration planning for mixed voice and fax estates, including port sizing, T.38 verification steps, and a phased cutover plan that keeps business-critical document transmission uninterrupted.
Redstone Systems, Inc. founded in Delaware, USA in December 2002, has been the ODM vendor for many well-known communications companies, serving the Southeast Asian market. In 2020, Redstone Systems will begin to return to the North America market with its self-developed brand.
Redstone has a complete product line of intelligent voice gateways, providing IP-PBXs, analog VoIP gateways (FXS/FXO), digital VoIP gateways (E1/T1), border appliances, and session boundary controllers (SBCs).
With advanced technology in digital signal processor (DSP), speech coding and speech processing, as well as efficient operational tools such as cloud remote management, auto provisioning, Redstone gateways are widely used in markets of enterprise communications, cloud communications, call centers, operators’ IMS/SIP trunks, bringing users friendly, efficient and reliable communication experience.
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