After fall, iPhone 4 doesn't transmit sound

After falling on its top right corner, my iPhone 4 occasionally stops transmitting sound. It makes and receives calls, but person at the other end cannot hear a sound.

Sometimes it gets back to normal, in particular after good shaking. Looks like a contact somewhere is lose.

Any ideas where I should look to repair it, or if there is a service in the UK that can do it for me (the phone is outside of warrantee or insurance)? It doesn't seem to be microphone, because recording app on the phone records sound even when the sound is not transmitted during calls.

Thank you