T.38 In VoIP is tricky. We run almost every codec in Kazoo that FS supports (711, 722, 729, all the way up to Siren64Khz and h.263 for video), and I can tell yo

T.38 In VoIP is tricky. We run almost every codec in Kazoo that FS supports (711, 722, 729, all the way up to Siren64Khz and h.263 for video), and I can tell you that t.38 has been the trickiest. A lot of the transformations layer7 firewalls apply to packets, particularly SIP packets, can cause unexpected side effects with an extremely time sensitive application like fax. In short, moving packets involves buffering on some level, and fax machines are intolerant of any jitter or delay. Therefore, delivering media in t.38 becomes an exercise in conflict mitigation. In summary, t.38 is active on a lot of providers who don't talk about it because heterogenous equipment might not play nice.