There are 2 main reasons for difficulty with sending faxes from an ATA. Firstly, network quality is very important - Jitter or packet loss will prevent faxes from working as a fax machine cannot tolerate an imperfect audio stream; This can be monitored with a network trace, and using a PC product such as “Wireshark” to analyse the results.
Secondly, and more usually, the ATA units have a large number of analogue configuration options, and some fax machines are intolerant of incorrect line settings, some suggestions follow, which apply to Sipura ATA devices:
(Set on the “Regional” settings tab of the ATA’s administration interface)
- FXS Port Impedence: The correct UK setting is “370+620||310nF” - For an American device it is “600”
- Caller ID Method: ETSI FSK With PR(UK)
- Caller ID FSK Standard: v.23
- Fax Passthru Method: ReINVITE (Asterisk does not currently support NSE, but this setting should be unimportant)
- Disable any Echo Cancelling and silence supression options, and/or set to disable EC on FAX tone detection
Once the above is set, it may then be necessary to modify the input and output gain settings. Using -6 in both fields is usually the best, but values of 0, -3 and -6 are usually worth trying in various combinations. Set and save a value, then try sending several (perhaps 10) faxes to see what the success rate is, then change one setting and re-test until the results are acceptable. It is usually possible to get a FAX machine working using this method.