I believe the main problem is with Packet Data interupts.Make sure config "When needed" and if necessary delete apn and disable - then back in reput "GPRS" and enable - then restart phone - just...

Also you may have another sip prog like Trufone [takes default as it's precedence][go offline helps],Eqo,Tringme etc
And may have uninstalled them and "VoIPAudioSrvc.sis" which should have installed...

I use Vodafone, definately not T-mobile
One works the latter doesnt.
Sometimes get same message "no internet...... "
Can usually get it back by "always on in sip config""automatic in internet tel"...