FWIW, AT&T will do exactly the same for their FLEX offering, you need to complete a 2000 page “Self Certification” course to use anything but Cisco and Nortel UA’s even if they are 10 years old (OK, I’m exaggerating, but FreePBX is NOT acceptable), pragmatically many people just change the UA to something in
http://www.voip-info.org/wiki/view/SIP+user+agent+identification