Additional info from FS console:

2013-05-23 10:30:40.402324 [DEBUG] mod_sofia.c:642 SOFIA EXCHANGE_MEDIA
2013-05-23 10:30:40.402324 [DEBUG] switch_core_session.c:923 Send signal sofia/external/myusername@my.office_pbx.ip [BREAK]
2013-05-23 10:30:40.402324 [DEBUG] switch_core_session.c:923 Send signal sofia/external/caller_number@my.fs.ip [BREAK]
2013-05-23 10:30:40.422955 [DEBUG] switch_core_io.c:1365 Engaging Write Buffer at 480 bytes to accommodate 320->480
2013-05-23 10:30:40.450958 [DEBUG] switch_core_io.c:1365 Engaging Write Buffer at 320 bytes to accommodate 320->320
2013-05-23 10:30:40.662295 [WARNING] switch_core_media.c:1370 Asynchronous PTIME not supported, changing our end from 0 to 20
2013-05-23 10:30:40.662295 [DEBUG] switch_core_media.c:1741 Already using PCMA