LetterAgent
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Internet faxing, usually called FoIP (fax over IP), carries your document over the internet instead of an analog phone line. Your file is split into packets, travels over IP, and is converted back into fax tones before reaching the receiver. Two approaches do this work: T.30-style audio carried over VoIP, and T.38, a protocol built specifically for fax.
Traditional fax runs on the T.30 protocol: the sending machine calls the receiving machine over an analog line, the two trade a series of chirps (the handshake), agree on a speed, and then send the page as tones. That works because an analog line is one continuous, unbroken stream of audio.
T.38 takes a different route. Instead of carrying the audio itself, it reads the fax tones, converts them into data packets, and rebuilds the tones at the far end. Because it is data rather than audio, T.38 can send each packet more than once: a dropped packet gets replaced by its duplicate instead of corrupting the page. That redundancy is the whole reason T.38 survives networks where T.30-over-VoIP dies.
| Method | How the fax travels | Reliability | Typical setup |
|---|---|---|---|
| Analog line (T.30 over copper) | Fax tones travel as one continuous audio stream | Very reliable | A fax machine and a landline |
| T.30 over VoIP (pass-through) | The same tones, carried inside voice packets | Fragile: jitter and compression corrupt tones | A VoIP adapter with a fax machine |
| T.38 | Tones converted to data packets and rebuilt at the far end | The most reliable internet option | A T.38-capable gateway or fax provider |
| Online fax service | You upload the file; the provider handles the fax leg | Reliable: no hardware or line at your end | An app, email, or an AI assistant |
Voice codecs like G.729 compress audio by discarding frequencies humans barely notice, and fax tones happen to live in those discarded frequencies. Even with uncompressed G.711, the jitter of a busy network can stretch or squeeze a tone past what the receiving machine recognizes, and the fax gives up and retries from the top.
Then there are the small sabotages people forget about: a call-waiting beep in the middle of a transmission resets the call, and echo cancellation on some adapters trims the leading edge of every tone. This is why a fax that sails through on a copper line can stall halfway on a VoIP adapter, and why providers that take fax seriously run T.38.
When you send a fax by asking your AI assistant, you never touch a protocol. You provide the document and the destination fax number; the service converts your file and hands it to a fax provider that sends it over a managed fax network. LetterAgent sends outbound faxes to US and Canadian fax numbers only, with no inbound faxes.
Fax with LetterAgent is currently in testing: the flow is a per-fax quote with the page count verified, you approve the exact total, you pay once through Stripe, and the fax is sent. You can ask for a quote today, but do not plan around a fax that must go out at an exact minute: treat it as testing until the service says otherwise.
That handshake chirp is a negotiation, and speed is the main item on the table. The sending machine offers its fastest speed, the receiver answers with the fastest it can handle, and they settle on the highest speed both support. On a clean line that means a page in well under a minute; on a noisy line they fall back step by step, 14,400 down to 9,600 and below, trading speed for reliability.
This is also why a fax sometimes seems to restart: if errors pile up mid-page, the machines renegotiate and resend from the last confirmed point. Error correction mode (ECM), which most machines have on by default, breaks each page into blocks and re-sends only the damaged ones. Turning ECM off can get a fax through on a terrible line, at the cost of the occasional garbled line printing without correction.
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.
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