I got my iPhone 4 from Verizon. My wife claims when she talks to me I keep dropping in and out. I know at and t had this problem. I can hear her fine and she's got sprint. Maybe her phones the problem?
In addition to above comments, mine might be overly simple: how many bars of signal are you seeing on the phones? Very weak signal strength (location, location, location...!) means a lot.
It could be her phone?? Please check out my app reviews [ame=http://www.youtube.com/watch?v=NNkYeayUqNM]YouTube - ‪Best Free Apps For iPhone - 25 May 2011‬‏[/ame]