When is an iPhone not an iPhone? When it won't function as a phone. By which, I mean, it cannot be trusted to receive phone calls.

In the past few weeks - that I've noticed anyway, although it could have been for much longer - my iPhone 3G will regularly miss incoming phone calls. Someone calls me, they're sent straight to answerphone. And this despite me being in a location which indicates good signal strength, typically the office, my home, or somewhere on the commute in between.

According to someone I spoke to at O2 Customer Support today, they have had many people calling with the same problem with their iPhones. Google around and you'll see various forums filled with people similarly bemused by why their iPhone is randomly refusing to receive telephone calls.

My first thought was that my handset had just developed a fault, as I'd only been troubled by this bizarre phenomenom recently. Something's gone awry with the hardware; these things can happen.

But no, O2 suggested that I should wait for iPhone OS 3, maybe that might fix it, because the iPhone cannot negotiate incoming calls (probably SMS txts too) when the phone is receiving wireless data.

And you don't even have to be surfing at the time. Like most people, I receive email through the iPhone, with a ‘push' service which means the iPhone is checking-in to a mail server regularly every hour. It seems those are the moments when if you're phoned, your call is deflected as if the handset was switched off.

I hate to repeat it, but let's just go over the situation again: the iPhone 3G cannot accept a phone call when you're online. It cannot field voice and data at the same time? Surely this can't be the case.

What's more, if the caller doesn't leave a message, you'd never even know they called, as you get absolutely no notification of a missed call.

Take away the 'i'...

A solution I was offered, known to fix this tragedy, is to delete the APN details in the iPhone's settings. At which point you lose all data connectivity, and your iPhone becomes a... Phone.

The impotence of being unavailable to callers was really brought home to me a few weeks ago.

An elderly neighbour was taken sick, and I had to call out a doctor and social services for emergency help. Outgoing calls, fine. But when the emergency services called me back to confirm crucial details such as address, my calls were neatly deflected to the answerphone. Every time

Times like that - that's when you realise how helpless it feels to have a phone that isn't a phone.

I've asked Apple and O2 if there's any official line on this situation. At this moment I'm awaiting for their story. I'll write an update when I hear more.

Has this happened to you? Let us now in the Mobile World forum.