As dicko said not all providers support T.38 so you have to make sure yours does...
I know that SipStation supports it and Flowroute supports it...
I had found another one but unfortunately for some reason it's as unreliable as G711, maybe even worse...
In the case of Flowroute I know I had to allow re-invites for it to work and I have never tried SipStation...
There are also providers which provide you with their own fax solution like VoIP.ms but as far as I know it is stil beta...
Good luck!
Nick