Can't hear caller (and vice versa) on C500

[Guest Gagging Ferret]

Please help!

I sold the phone on EBay, unlocked, all confident that it was a good buy for someone, then she e-mails me to say it's faulty :D

I've got it back now and she's right - I made a call from it and found that the caller couldn't hear you and you couldn't hear them.

I've done a hard reset and we're half way there - I could hear the caller, but the caller couldn't hear me.

I've tried it with T-Mobile, Orange and 02 sims and the same thing happens. There are two factors here, as the phone was fine when I got it. Is it possible that something went wrong during the unlock procedure? Also, there was dust behind the screen and I cleaned it out, following the instructions I found on this site. Other than that, it's as I bought it.

Any idea as to what's happened please?

Thanks ;)