What do you mean by sync? If you're talking about the normal wireless reconcile that happens when emails or changes to your BES calendar or contacts, that data connection stops automatically when voice comes in. If it hasn't always done this, try pulling the battery, could just be some instability of the OS. If this has always happened, make sure you're on the latest OS available for your phone, could have been a bug.
If you're talking about a sync you run from DM, you can't stop that to take a call.
