I’m watching the incoming call on the asterisk channels page, it shows the G.726 codec in use. I’m also forcing the G.726 codec on the trunk and disallowing all others.
As well, I’m using an online fax provider to send the fax, which reports when complete that it negotiates a fax rate of either 9600 or 12200 baud. When using G.711 it would always negotiate the highest 14400 baud.
From what I’m reading, G.726 should be able to carry fax.
“The G.726 codec was, however, specifically designed to be able to carry medium speed modems, such as the V.29 modem used for FAX.”
Source: http://www.soft-switch.org/foip.html
“The dynamic switch allows the speed of voice band modem transmitted by G.726 at 32 kbit/s to be boosted to 9600 bit/s. At 40 kbit/s, modem speeds up to 14400 bit/s are possible.”