Crosstalk and Music(?) On International Calls

[Shadragon]

We have a user who calls overseas to London, UK with regularity. He keeps saying the people he is calling can hear crosstalk and once commented that a guitar could be heard. The voice quality is reported to be spotty and broken at their end.

The user can hear some breaking up of the call, but his in-house, local and calls to North America are trouble free. I've changed up his codecs with little effect. Any fault finding tips I can try?

Thanks.