What do you mean gets muted? Does the volume just decrease or do you accidently hit the mute button? If it's just a volume issue, you might want to check that you're keeping your battery fully charged and that you don't accidently hit the volume rocker when you're on the phone. If you accidently are hitting the mute button you could try not pressing it directly to your ear or using a handsfree set/headphones.
Or does the phone just mute itself automatically? Then it could be a software issue.... If you can give a little more detialed discription, I'll ask around and see what we can come up with!