T38 faxing

I’ve been trying to setup T38 to work with my provider (Broadvox), can anyone give me any pointers? I’ve enabled t38pt_udptl=yes in the sip.conf and also on each extension. I’ve tried calling from Kapanga and Zoiper and neither work. I’ve tried multiple combinations of canreinvite. Can anyone point me in a working configuration for Asterisk 1.4?