About ten days ago I wrote about a bizarre issue with my iPhone 3G, where calls were being randomly but consistently diverted to answerphone. And this would typically be with the phone sat on my desk next to me in the office, merrily showing a 3G connection flag and full signal strength.

After information received from calling O2 Customer Support, and reading some ideas being bandied around on various forums from people experiencing similiar issues, I was led to believe that there could be an issue whereby the iPhone 3G couldn't juggle data and voice at the same time. If someone calls you while you're surfing, or even if the phone is just automatically polling to check for mail in the background, then any incoming calls won't reach you.

Apple still hasn't replied to my query to assert whether this could be true or not; but O2 did get back to me, and set about investigating the issue.

It turns out the situation is more complex than a design fault with the iPhone 3G, or even with smartphones in general.

There are issues with data and voice conflicting, but that may be more relevant to pre-3G networks. Such as EDGE - which the iPhone 3G also uses when it can't find a steady 3G signal to latch onto...

So O2's network performance engineers put a trace on my phone, keeping a log of my network usage over the course of several days. A week later, my claims for missed calls were all substantiated by the call logs. And this is where it gets complicated, as there's more than one problem at stake here.

At the PC Advisor office, on the Euston Road and directly opposite the British Library, there would seem to be something of a deadspot as far as 3G communications go. We're on the cusp of two 3G cell sectors, plus a third 2G micro-cell. And with no single cell taking dominance here, the phone is constantly going off the network, making me unreachable by phone. Not very helpfully, there's no indication of this on the handset.

The oxygen company's Network Performance Manager visited the office this morning, to test the situation first-hand. Sure enough, there's something amiss in this area, which could stretch to the huge pool of O2 users in and around the Euston/St Pancras/Kings Cross area. Or it could just be at 101 Euston Road. And the Pret á Manger coffee shop below. The company's best engineers are now on the case.

Meanwhile, my call missages at home in south London have been attributed to being in a fringe area, with poor coverage in a zone between cells. Here, there's a more credible reason, as signal strength is showing as somewhat low on the iPhone handset, and I can see the network constantly switching between round blob (GPRS), E for EDGE, and 3G data connections, all as I wander around my far-from-expansive top storey flat.

Unfortunately for would-be callers, it can take 45 seconds for the phone to be available again after every network switch. Which explains why I'm frequently domestically incommunicado.

I'm living just outside O2's key London 3G zone, an area encircled by the South and North Circular Roads. At the end of last year O2 rolled out new network infrastructure inside this zone to keep up with the escalating demand, principally from the iPhone 3G and USB 3G modems. This area is deemed the city's most important ground, given its high population density and number of tech-savvy residents and visitors, and should now get the best network coverage.

Ironically, I live just south of the South Circular, between two of the most powerful transmitter masts in the country, serving up a sizable chunk of the south-east of England.

Unfortunately for any continued chatting pleasure, these towers are not your usual mobile phone relays. They're more focused on analogue television, Freeview TV, FM and digital radio broadcasts, than on my personal chatting...

Once again, I'll update here when one or both of these iPhone 3G blackout zones lighten up at all.