I get "Call disconnected" to anything but a phone number at my work.
m=audio 7078 RTP/AVP 96 18 8 0 13 97 c=IN IP4 81.187.82.182 a=rtpmap:96 iLBC/8000 a=fmtp:96 mode=30 a=rtpmap:18 G729/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:0 PCMU/8000 a=rtpmap:13 CN/8000 a=rtpmap:97 telephone-event/8000