There is a bug in the os7000 firmware prior to 4.80 I think where when when you disable a codec in the sip trunk settings, the system still offers that codec in the sip header.

This causes calls to ms lync for example to fail, as it wants g.711u only.

This may be your issue.

Are the handsets dgp or ip?
If ip you will need to change to g711 in the ip phone settings as well as the sip trunk settings.