1. Do you have a case on the phone? It might be interfering with your speaker/mic.
2. Have you paid any attention to your signal strength during a call?
I had this problem when I first got my Tour. Maybe the same solution will work for you.
The microphone is on the bottom left of the device as you are looking at the face. See it. When I'm talking on the phone I inadvertently covered the mic with my pinkie! I did this randomly so my voice was clear theb muffled suddenly.
Now that I'm aware of it I keep my finger off the mic.
Just a thought.
Posted from my CrackBerry at wapforums.crackberry.com
Have you ever connected your device to your pc/mac?
Different OS's have different radio files which dictate how your signal works. I'm hoping that this is a software issue for you which would make it a cheap fix. If the hardware is the problem, then that will cost money.