4S Incoming Call Audio Issue

[scientifics]

I have an issue that only happens with a very specific set of circumstances on my iPhone 4S. When I receive a call from one person (another VZW number) I can hear them, but they can't hear me. When we hang up and they try me again, everything is fine (except for a ~5 second delay before they can hear me). The issue only happens with this one person and when they are calling me. Sometimes everything is fine, sometimes I have the 5 second delay (before they can hear me, I can hear them the whole time), and sometimes they can't hear me until the call me back (I can always hear them). I've tried doing a restore to no luck. Any ideas on what else to try? And is this a VZW issue or an issue with my phone?

[soupcannon]

Maybe it's them not you