Well, I'll tell you one thing. My daughter departed our T-Mobile family plan for Verizon to get an iPhone. On MANY MANY of her calls to us, both to our cell phones and land lines both here and at work, I can barely understand what she's saying. Sounds like she's talking through bubbles. I believe this is an artifact of excessive compression. This NEVER happened on T-Mobile. So, IMHO T-Mobile should answer Verizon's commercials like this:
"Can you hear me now?"
"Yes, but I can't understand a word you are saying!"