[SR-Users] UAC call to AT&T

Hello. we use UAC module for trunk registration at kamailio. We have successfull routing and calling through our trunks to many providers and have just one problem with AT&T endpoints. When callee at AT&T endpoint hook call (I see OK from provider as response to our INVITE from AT&T endpoint) and then kamailio sends CANCEL to endpoint, but then kam sends ACK to OK... So after this strange scenario I successfully hear invite to leave voice mail to AT&T endpoint... What I saw: I look at codecs- at OK i See ulaw codec, so at my INVITE ulaw presents too. With other providers calls succesfully established. Does Anyone have an experiense with calling to AT&T from Kamailio. P.S. calls through our trunk to AT&T from asterisk establishes succesfully too.