Siemens S66 / S65 / SP65

I got my S66 about two months ago, and have noticed that quite frequently during a call everything will go silent and I will miss what the other person is saying on the other end for about 7 or 8 seconds, but will NOT drop the call. The reception strength doesn't vary, nor is there any static or anything. Has anyone else had an issue like this before? Is it more likely that it is the phone or Cingular's service? Thanks for any comments or help.