Xperia X10 :: Callers Can't Hear Me When Answering

I am having an intermittent problem where callers can't hear me for 6-10 seconds when they call. This is a t best embarrassing and frequently they hang up. As I say the problem is intermittent - sometimes the phone answers as usual. Interestingly, I can hear them talking (and usually saying 'hello? hello?) but they can't hear me and I just keep saying hello until they hang up or until the sound kicks in and we can converse.

I did a factory reset with no improvement, and have also downloaded the PC updater and updated the system with no improvement. Then, I spoke to a friend the other night who described an identical problem with his X10.