If you are syncing your iPhone and want to make a phone call, you'll have to cancel the sync.
However, if you're already on the phone and want to sync and continue talking, the iPhone won't drop the call. So, syncing while talking is possible if you need to do it; just start the call before syncing.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090423141104947