Cisco 7902 and 7912 phone calls breaking up

I am expieriencing calls breaking up on two models of Cisco phones, the 7902 and the 7912. I have 7940’s and 7960’s that work fine on the same 3560 POE router. They are all being ran by Cisco CallManager ver. 4.1 and each model phone has the latest software versions. Has anyone expierenced this problem?