Receiver not working on iPhone

So I bought an iPhone 4 broken so that I could fix it. I replaced the screen and everything works perfectly, except one thing, I cannot hear anything out of the receiver or the speaker during a call. However, when I Facetime with it I can hear out of the speaker and it works fine. I put the mother board in another phone (with a receiver I know works) and I had the same problem. I can only assume it is on the logic board then.
Anyone know of a quick fix for this? I did do a hard reset(DFU) and I still had the same problem.

Have you done a hard reset by holding the sleep/wake button and the home button. Or doing a clean restore?

The iPhones quite often have this problem, usually the quick fix is to plug in and unplug a headset or headphones a few times to kick it back into the right track.

- Simon