If you haven't done so already, first check with your operator if there's any known network problem in your area. Also check if this problem occurs regardless who you're talking to (might be a problem on the other end).
If that's that not the problem, try the phone on GSM only and WCDMA only for a while to see if it's a problem with one of the networks. You can set this under Settings -> More -> Mobile networks -> Network mode.
If you still have problems after this i recommend that you try the phone with another SIM-card and/or contact a service location to have the phone investigated.
