How to set up internet fax using T 38 or fax over IP
The protocol, the SIP trunk relationship, the MFP configuration, and the alternative cloud fax services — for offices migrating from analog fax lines to IP-based fax.
Spanish telecom providers have largely retired the public analog phone network in favour of fibre-and-VoIP delivery. The traditional analog fax line many offices used for decades is either gone or running on top of VoIP infrastructure via an analog telephone adapter. Modern fax over the same VoIP infrastructure requires T.38, the ITU standard that carries fax tones as discrete IP packets rather than compressed audio. This guide covers the three ways to put T.38 into practice and the configuration each requires.
Three approaches to internet fax
MFP speaks T.38 natively
The MFP includes a T.38 fax module that registers to a SIP trunk and exchanges fax directly without an analog gateway. Cleanest setup but requires native T.38 support on the device.
Analog MFP via T.38 ATA
An analog telephone adapter sits between the MFP's analog fax port and the SIP trunk. The ATA converts analog tones to T.38 packets and back. Works with any analog-fax-capable MFP.
Email or API fax service
A cloud fax service receives email or API submissions and delivers as fax to the destination. The office MFP plays no role in fax delivery. Lowest infrastructure investment.
Approach 1: Native T.38 on the MFP
The cleanest setup is the MFP speaking T.38 natively to a SIP trunk. Enterprise MFPs from major vendors typically support T.38 — verify on the device's fax module spec sheet before assuming it does.
Verify native T.38 support
Check the MFP's fax module specifications. Look for "T.38 over SIP" or "IP-Fax" with T.38 listed. If only "G.711 fax pass-through" is supported, the device cannot use approach 1 reliably and should follow approach 2 or 3 instead.
Obtain a SIP trunk with T.38 support
Coordinate with the telecom provider for a SIP trunk that supports T.38 fax. Spanish providers offering this include Movistar (Centralita Conecta), Vodafone (One Net), Voipfuture, Acumbamail Voice, and various ITSP specialists. Provider provides: SIP server address, authentication credentials, fax channel allowance.
Configure the SIP account on the MFP
Open the MFP web admin → Fax → SIP/T.38 settings. Enter: SIP server sip.provider.es, registration username, password, realm, codec (T.38 priority). Save and verify the device registers successfully.
Set T.38 transport parameters
Transport: UDP (most providers) or TCP (some prefer). T.38 version: 0 or 1 depending on provider. Redundancy: enable T.38 redundancy packets if the network has any packet loss. ECM: keep enabled.
Test send and receive
Send a test fax to a known destination, then have a sender fax back. Both directions must work before declaring the setup complete. Examine the SIP/T.38 logs if either direction fails.
Approach 2: ATA-bridged T.38
The pragmatic path for offices with existing analog-fax MFPs that lack native T.38 support. An ATA (Analog Telephone Adapter) such as the Cisco SPA122, Grandstream HT813, or Patton SmartNode bridges the MFP's analog fax port to a SIP trunk.
Choose a T.38-capable ATA
Not all ATAs handle T.38 reliably. Recommended: Cisco SPA-series, Grandstream HT-series (HT813 specifically), Patton SmartNode. Avoid consumer-grade ATAs that handle voice well but corrupt fax tones.
Connect the ATA between MFP and network
The ATA has at least one FXS port (analog line out to the MFP) and one Ethernet port (network connection). Connect MFP's LINE port to ATA's FXS port with an RJ-11 cable. Connect ATA's Ethernet to the office network.
Register the ATA to the SIP trunk
Access the ATA's web admin (default IP varies by model). Enter SIP credentials provided by the trunk vendor. Enable T.38 mode on the FXS port and disable voice-priority codecs (G.729 in particular).
Configure ATA fax-specific settings
FXS port mode: Fax (not Voice). T.38 fax relay: enabled. T.38 packet redundancy: enabled. Disable jitter buffer adaptive mode for the fax port — fax requires fixed timing.
Configure the MFP for standard analog fax
The MFP sees the ATA as a normal analog phone line. Configure fax settings as for a PSTN line (see the standard fax line setup tutorial). Often setting fax speed to 9600 bps maximum produces the most reliable results across imperfect IP paths.
T.38 protocol parameters
Port→negotiated dynamically via SIP INVITE
Max rate→14400 bps (V.17) — higher rarely needed
Redundancy→level 1 or 2 (forward error correction)
Packet rate→30 ms typical
Buffer size→200 bytes default
Fill bit removal→disable unless specifically required
Approach 3: Cloud fax service
Cloud fax services (eFax, Concord, RingCentral Fax, Movistar Fax Online) deliver fax functionality without the office maintaining any fax-specific infrastructure. The flow: users compose an email to a special address provided by the service, the service converts the email into a fax and sends it to the destination fax number. Incoming faxes arrive as PDF attachments to a configured email address.
The trade-off is per-page or per-month pricing (typical €0.05-0.15 per page or €10-40 monthly subscription) versus the per-line cost of a SIP trunk plus the MFP fax module that would otherwise be required. For offices sending fewer than 50 faxes per month, cloud fax often costs less than the alternatives.
Approach comparison
| Factor | Native T.38 | ATA bridge | Cloud fax |
|---|---|---|---|
| MFP requirement | T.38-capable | Any analog-fax-capable | None (email/web) |
| Hardware investment | None additional | ATA €100-300 | None |
| Per-fax cost | SIP trunk per-minute | SIP trunk per-minute | Service per-page |
| Setup complexity | Moderate | Moderate-high | Low |
| Operational reliability | High | Medium-high | High |
| Best for | Modern MFP fleet | Existing analog fax MFP | Low volume / new offices |
Common T.38 setup issues
What goes wrong
- Fax connects but pages garbleThe SIP trunk fell back to G.711 instead of negotiating T.38. Check the provider supports T.38 and the device's SIP INVITE includes T.38 in the SDP.
- T.38 negotiation fails entirelyNAT or firewall is rewriting SIP packets. Use a SIP-aware NAT or configure the SIP trunk for outbound proxy mode.
- Some destinations fax fine, others failThe other end may not support T.38 — the gateway between providers degrades to G.711. Reduce fax speed on the MFP to 9600 bps as a fallback.
- Random page drops mid-transmissionNetwork jitter exceeds T.38 buffering. Enable T.38 redundancy or increase buffer size in the ATA settings.
- SIP registration drops periodicallyThe provider's SIP server expects keep-alive at a specific interval. Configure registration refresh below the timeout (typically 60-90 seconds).
- Inbound faxes never arriveThe SIP trunk's DID may not route fax to the device. Coordinate with provider on inbound routing — sometimes a separate inbound DID is required for fax versus voice.
When to skip T.38 entirely
For offices receiving fewer than 30 faxes per month, the cloud fax service approach (approach 3) is usually the most operationally efficient. The infrastructure overhead of maintaining T.38 — SIP trunk, T.38-capable MFP or ATA, troubleshooting capacity in IT — exceeds the per-page cost of cloud fax at low volume. For offices receiving hundreds of faxes monthly, native T.38 amortises better and provides direct integration with the office MFP workflow.