IPhone :: 2G Has Strong Signal But Somtimes Cannot Make Calls

I recently got an iPhone from a friend that has a very strange fault. It can show a very strong signal (5 bars) but will intermittently not make or receive calls and texts. For example just with the phone sat here on my desk one min it will make outgoing calls and the next it just sits at "calling" with no ring tone for about 45 secs before moving to "Call Failed". Incoming calls sometimes work and sometimes go straight to answer phone. Outgoing texts sometimes fail to send and sometimes work.Just for a bit of background this is an unlocked 8GB iPhone 2G on firmware 3.1.3. I've tried both Orange and Tesco (O2) sims and both have the same problem. The Wi-Fi and everything else work perfectly and the phone itself is in good physical condition. I have tested the sims in an 16GB iPhone 2G with identical firmware and that works fine.

So any ideas what it might be? I've kind of discounted the sims and the firmware which only really leaves a hardware problem. Could it be a problem with the aerial or would that show a weak signal strength? Is it relatively easy to replace and would you recommend it?