iPhone 4: no sound on outgoing calls

I have the same problem. No sound outgoing, but they can hear me when the speaker is on.

Try plugging a mini plug in and out of the top headphone jack. If that plug is malfunctioning, it will disable the ear-piece speaker

I now have the same problem after 5 months on good service.
Currently have an appointment reserved for tomorrow. Hopefully to get a replacement as I have seen on other forums this is a fault which can not be fixed by shop staff or over the phone and has occurred with 3's and 3G's.

Embarrassingly for Apple I have had to put my iPhone into call forwarding to my Blackberry!

I'll reply with an update.

I am having the same problem but I have to forward to a land line.

Apple please help us poor smucks

As a follow-up to my earlier post:

"I now have the same problem after 5 months on good service.
Currently have an appointment reserved for tomorrow. Hopefully to get a replacement as I have seen on other forums this is a fault which can not be fixed by shop staff or over the phone and has occurred with 3's and 3G's.

Embarrassingly for Apple I have had to put my iPhone into call forwarding to my Blackberry!"

I attended my appointment, unfortunately a very busy day there.. so had to wait 50 minutes.. but hey..

Tech support guy was very good, did a simple check call on the phone to confirm what I was reporting, then ran a very quick diagnostic check on the phone just because that's what they're instructed to do.. then replaced it with a brand new unit.
These are the tech support supply which come in a black box. He reckoned that u/s phones do not get repaired due to cost etc which I can believe, maybe the cpu and ram may get salvaged but the rest simply gets recycled. Therefore suggesting that the replacement phone must be factory new, not a refurbished unit. It certainly doesn't look 2nd hand.
They register the new serial number as being your phone and the warranty continues as before.
Then all you do is update firmware and re-sync when you get home zzzzzzz

*I spent ages trying to find my receipt for my phone, ready to take to my Genius appointment...... as it happened I didn't need it as they already have your details in their system.

All in all a very good response by Apple.