Nokia n73 problems since being unlocked. sound quality on phone calls is awful

[ben1990]

I will try and shorten this down as much as possible. Basically i originally had a nokia n73 on a t-mobile contract. At the end of my contract i decided to get an upgrade and also get my old nokia n73 unlocked. I then gave this phone to my girlfriend who used it on the o2 network. She complained that the phone worked extremely slow which it had never dne before it was unlocked. Recently the phone would no longer send text messages and they kept failing. I managed to solve this problem by updating the firmware and then reseting the phone. Since this though the sound on making calls is terrible for the other person. It can be extremely crackly to the other person who my girlfriend is calling and also she struggles to hear the other person. This is causing the phone to be unusable and i was hoping for any suggestions possible from anyone. All help is much appreciated. Sorry for the post being so long but i felt i should include all the details so you could see my problem.