Here's the complete log (with debug) of the inbound call.
Spent my whole day trying to understand it.. without any luck.
2012-03-24 01:18:37.865450 [NOTICE] switch_channel.c:926 New Channel sofia/external/
1184049130@189.84.133.130:5060 [4898b56c-754f-11e1-af7e-812af071ddc6]
2012-03-24 01:18:37.865450 [DEBUG] sofia.c:5532 Channel sofia/external/
1184049130@189.84.133.130:5060 entering state [received][100]
2012-03-24 01:18:37.865450 [DEBUG] sofia.c:5543 Remote SDP:
v=0
o=VoipSwitch 6106 7106 IN IP4 189.84.133.130
s=VoipSIP
i=Audio Session
c=IN IP4 189.84.133.130
t=0 0
m=audio 6106 RTP/AVP 18 101
a=rtpmap:18 G729/8000/1
a=fmtp:18 annexb=no
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
2012-03-24 01:18:37.865450 [DEBUG] sofia_glue.c:4879 Audio Codec Compare [G729:18:8000:20:8000]/[G7221:115:32000:20:48000]
2012-03-24 01:18:37.865450 [DEBUG] sofia_glue.c:4879 Audio Codec Compare [G729:18:8000:20:8000]/[G7221:107:16000:20:32000]
2012-03-24 01:18:37.865450 [DEBUG] sofia_glue.c:4879 Audio Codec Compare [G729:18:8000:20:8000]/[G722:9:8000:20:64000]
2012-03-24 01:18:37.865450 [DEBUG] sofia_glue.c:4879 Audio Codec Compare [G729:18:8000:20:8000]/[PCMU:0:8000:20:64000]
2012-03-24 01:18:37.865450 [DEBUG] sofia_glue.c:4879 Audio Codec Compare [G729:18:8000:20:8000]/[PCMA:8:8000:20:64000]
2012-03-24 01:18:37.865450 [DEBUG] sofia_glue.c:4879 Audio Codec Compare [G729:18:8000:20:8000]/[GSM:3:8000:20:13200]
2012-03-24 01:18:37.865450 [DEBUG] sofia_glue.c:4879 Audio Codec Compare [telephone-event:101:8000:20:0]/[G7221:115:32000:20:48000]
2012-03-24 01:18:37.865450 [DEBUG] sofia_glue.c:4879 Audio Codec Compare [telephone-event:101:8000:20:0]/[G7221:107:16000:20:32000]
2012-03-24 01:18:37.865450 [DEBUG] sofia_glue.c:4879 Audio Codec Compare [telephone-event:101:8000:20:0]/[G722:9:8000:20:64000]
2012-03-24 01:18:37.865450 [DEBUG] sofia_glue.c:4879 Audio Codec Compare [telephone-event:101:8000:20:0]/[PCMU:0:8000:20:64000]
2012-03-24 01:18:37.865450 [DEBUG] sofia_glue.c:4879 Audio Codec Compare [telephone-event:101:8000:20:0]/[PCMA:8:8000:20:64000]
2012-03-24 01:18:37.865450 [DEBUG] sofia_glue.c:4879 Audio Codec Compare [telephone-event:101:8000:20:0]/[GSM:3:8000:20:13200]
2012-03-24 01:18:37.865450 [DEBUG] sofia_glue.c:5000 Set 2833 dtmf send/recv payload to 101
2012-03-24 01:18:37.865450 [DEBUG] switch_channel.c:2848 (sofia/external/
1184049130@189.84.133.130:5060) Callstate Change DOWN -> HANGUP
2012-03-24 01:18:37.865450 [NOTICE] sofia.c:5813 Hangup sofia/external/
1184049130@189.84.133.130:5060 [CS_NEW] [INCOMPATIBLE_DESTINATION]
2012-03-24 01:18:37.865450 [DEBUG] switch_channel.c:2871 Send signal sofia/external/
1184049130@189.84.133.130:5060 [KILL]
2012-03-24 01:18:37.865450 [DEBUG] switch_core_session.c:1182 Send signal sofia/external/
1184049130@189.84.133.130:5060 [BREAK]
2012-03-24 01:18:37.865450 [DEBUG] switch_core_state_machine.c:362 (sofia/external/
1184049130@189.84.133.130:5060) Running State Change CS_HANGUP
2012-03-24 01:18:37.865450 [DEBUG] switch_core_state_machine.c:602 (sofia/external/
1184049130@189.84.133.130:5060) State HANGUP
2012-03-24 01:18:37.865450 [DEBUG] mod_sofia.c:469 Channel sofia/external/
1184049130@189.84.133.130:5060 hanging up, cause: INCOMPATIBLE_DESTINATION
2012-03-24 01:18:37.865450 [DEBUG] mod_sofia.c:534 Responding to INVITE with: 488
2012-03-24 01:18:37.865450 [DEBUG] switch_core_state_machine.c:47 sofia/external/
1184049130@189.84.133.130:5060 Standard HANGUP, cause: INCOMPATIBLE_DESTINATION
2012-03-24 01:18:37.865450 [DEBUG] switch_core_state_machine.c:602 (sofia/external/
1184049130@189.84.133.130:5060) State HANGUP going to sleep
2012-03-24 01:18:37.865450 [DEBUG] switch_core_state_machine.c:393 (sofia/external/
1184049130@189.84.133.130:5060) State Change CS_HANGUP -> CS_REPORTING
2012-03-24 01:18:37.865450 [DEBUG] switch_core_session.c:1182 Send signal sofia/external/
1184049130@189.84.133.130:5060 [BREAK]
2012-03-24 01:18:37.865450 [DEBUG] switch_core_state_machine.c:362 (sofia/external/
1184049130@189.84.133.130:5060) Running State Change CS_REPORTING
2012-03-24 01:18:37.865450 [DEBUG] switch_core_state_machine.c:662 (sofia/external/
1184049130@189.84.133.130:5060) State REPORTING
2012-03-24 01:18:37.865450 [DEBUG] switch_core_state_machine.c:79 sofia/external/
1184049130@189.84.133.130:5060 Standard REPORTING, cause: INCOMPATIBLE_DESTINATION
2012-03-24 01:18:37.865450 [DEBUG] switch_core_state_machine.c:662 (sofia/external/
1184049130@189.84.133.130:5060) State REPORTING going to sleep
2012-03-24 01:18:37.865450 [DEBUG] switch_core_state_machine.c:387 (sofia/external/
1184049130@189.84.133.130:5060) State Change CS_REPORTING -> CS_DESTROY
2012-03-24 01:18:37.865450 [DEBUG] switch_core_session.c:1182 Send signal sofia/external/
1184049130@189.84.133.130:5060 [BREAK]
2012-03-24 01:18:37.865450 [DEBUG] switch_core_session.c:1382 Session 5 (sofia/external/
1184049130@189.84.133.130:5060) Locked, Waiting on external entities
2012-03-24 01:18:37.865450 [NOTICE] switch_core_session.c:1400 Session 5 (sofia/external/
1184049130@189.84.133.130:5060) Ended
2012-03-24 01:18:37.865450 [NOTICE] switch_core_session.c:1402 Close Channel sofia/external/
1184049130@189.84.133.130:5060 [CS_DESTROY]
2012-03-24 01:18:37.865450 [DEBUG] switch_core_state_machine.c:491 (sofia/external/
1184049130@189.84.133.130:5060) Callstate Change HANGUP -> DOWN
2012-03-24 01:18:37.865450 [DEBUG] switch_core_state_machine.c:494 (sofia/external/
1184049130@189.84.133.130:5060) Running State Change CS_DESTROY
2012-03-24 01:18:37.865450 [DEBUG] switch_core_state_machine.c:504 (sofia/external/
1184049130@189.84.133.130:5060) State DESTROY
2012-03-24 01:18:37.865450 [DEBUG] mod_sofia.c:374 sofia/external/
1184049130@189.84.133.130:5060 SOFIA DESTROY
2012-03-24 01:18:37.865450 [DEBUG] switch_core_state_machine.c:86 sofia/external/
1184049130@189.84.133.130:5060 Standard DESTROY
2012-03-24 01:18:37.865450 [DEBUG] switch_core_state_machine.c:504 (sofia/external/
1184049130@189.84.133.130:5060) State DESTROY going to sleep