LetterAgent
Mail physical letters by just asking your AI assistant.
Sometimes, but it often fails. VoIP compresses voice calls in ways that garble the tones fax machines use to talk to each other, so pages arrive garbled, cut off, or not at all. A dedicated analog line works best; if you only have VoIP, an adapter with fax-friendly settings or an online fax service is a more reliable route.
A fax call is not a conversation. Two machines trade precise audio tones to agree on speed, then send each page as a stream of sound. The system was built for old analog phone lines, which carry those tones cleanly and steadily. VoIP turns voice into data packets, squeezes them to save bandwidth, and reassembles them at the other end. Human ears barely notice the small gaps and distortions this creates. Fax machines notice them a lot: a lost packet can corrupt a line of a page, and jitter, the uneven arrival of packets, can make the two machines fall out of step and drop the call partway through.
The codec matters too. Some VoIP setups use a codec called G.711, which passes audio through with little compression and gives fax its best chance. Many providers default to a compressed codec instead, which is fine for speech and rough on fax tones. Add call waiting beeps, echo cancellation, or a congested home internet connection, and a fax that worked yesterday can fail today for no obvious reason.
| Setup | How fax usually behaves |
|---|---|
| Analog phone line | The most reliable option; tones travel undistorted |
| VoIP with G.711 and a fax adapter | Often works, but still less steady than analog |
| VoIP with a compressed codec | Frequent failures, garbled pages, dropped calls |
| Online fax service | Skips the voice line entirely; the provider handles transmission |
T.38 is a protocol that carries fax signals as data instead of audio, so the tones never pass through voice compression at all. When both your adapter and your VoIP provider support T.38 properly, fax reliability improves a lot. The catch is that both ends must support it, and many consumer VoIP plans do not, or support it only partly. If your provider cannot confirm T.38 support in plain terms, assume you do not have it.
Start with a wired internet connection for the adapter, since Wi-Fi adds jitter. Turn off call waiting on the fax line, and turn off any silence suppression or echo cancellation your provider lets you adjust. Set the fax machine to its slowest speed, which gives the connection more room for error, and disable error correction only as a last resort: turning it off can let a damaged fax complete, but the pages may arrive with lines missing. Test with a short fax to a number you control before trusting the setup with anything important.
If the fax is long, time-sensitive, or carries sensitive content, do not gamble on a flaky line. A failed fax that looks sent, with a confirmation page for a partial document, is worse than a clear failure. In that case use an analog line if one is available, or an online fax service. Sending a fax through LetterAgent works by asking your AI assistant: you get a per-fax quote with the page count verified, approve the exact total, and pay once through Stripe. Fax is available in testing, and only US and Canadian numbers are supported. Because the service handles transmission for you, your own phone line never enters the picture.
Ready to fax it? Fax with LetterAgent: describe it to your AI assistant, approve the exact quoted price, and it is sent.
LetterAgent is a working name. Prices include print, postage, and our service fee. US prices in USD, Canadian prices in CAD (Canadian prices exclude applicable sales tax, added at checkout). Currently in private testing.
Need help? Ask your assistant: it can check a job's status any time. If it ever can't handle something: hello@getletteragent.com