Impossible Connection? LG-VX8300

A friend of mine has been sending me text messages on my cell and I receive them but I'm unable to respond to them.

Shes also tried calling my cell and it rings for her phone, but not on mine. When I try and send her a text message back I get:

"*Duplicate* From: Message to XXXXXXXXXX) not delivered - Invalid destination adress."

Whenever I'd call her cell I'd get "Beep---Caller you are trying to reach has not set up their voice mailbox. . . . . .goodbye."

I don't think its my end thats messing up is it? ; ;

Any help would be appreciated.

At first I thought it was my own cell, so I tried calling my phone on a ground line. Worked just fine.