Mobile, Sony Ericsson K800i?

No sound, can't hear a ringing tone but the phone does connect to the number called. Answerer can't hear me and I can't hear them unless I switch to 'speaker'. Have checked all the relevant settings and done a default reset, to no avail - problem is still there!

I see that this is a problem experienced by others but have yet to see anyone who knows how to successfully resolve it.