I noticed at the weekend that my iPhone 4 was having trouble with voice calls. Incoming calls would sometimes hang up when I answered them, outgoing calls would take a very long time to connect (or freeze), and when I did get through to someone the call quality was extremely low (lots of stuttering and cutting out).

After trying just about everything recommended on various help pages (including reseating the sim card, refreshing the network settings, and restoring the phone), I still could not get it to work properly. I then tried turning off the 3G and, lo and behold, the phone worked absolutely fine! Voice calls were clearer than ever!

I contacted Tesco Mobile hoping they might be able to shed some light. They said there are no faults with the network and have arranged for me to return my phone. That could take up to 10 days though, which I can't really afford.

Does anyone know what the problem is here? Does the fact that it works fine with 3G disabled suggest that it is not a hardware problem?

It can mean there is a problem with the network or with phone using the network. I'd try with another phone (not iphone) and then swap the sims as well to see if the problem moves with the sim or not.

Oh dear, that's disappointing.

Does anyone know if there are any speedier ways of getting this resolved than sending it to Tesco Mobile for 10 days? I might be able to make it into an Apple Store at the weekend - can they replace the phone there and then if faulty?

Yes you can take it to an Apple store directly and if it is faulty they will replace it on the spot. Might be an idea to book an appointment though, as weekends can be busy.