I have problem that incoming audio on a sip call doesn't work with my provider (voipfone.co.uk) although I do get ring tones (oddly US style rather than UK)

Outgoing audio seems to be fine does anyone have any clues as to how to progress this further? i.e. are there log files or packet captures I can run to get a clue? Should I be messing around and disabling codecs?

I've had a quick look through bugzilla but there doesn't seem to be anything directly like this.

BTW on the same wireless network my PC can make calls fine using Zoiper on windows.