Faulty Infineon chips could be to blame for some iPhone 3G owners' struggle to get a consistent signal when making calls on their phones, according to reports.

Apple's discussion threads are also full of users complaining at poor call quality - and these reports aren't just emerging in the US.

"I have had the phone for a couple of weeks and I haven't had any dropped calls until a couple of days ago. Now, at least 50 percent of my calls get dropped or I get a 'call failed' message when trying to make a call even though I have [a strong signal] on 3G," writes one iPhone owner on Apple's iPhone support forum, as reported by The Street. "The phone will also bounce from 3G to EDGE to 'No Service' and then back to 3G. The phone works fine if I turn off 3G, but that defeats the purpose of having a 3G iPhone."

Speaking to Cnet News.com, AT&T spokesman, Mark Siegel, said the company is looking into individual cases, but says it's not a wide problem: "How a device performs in individual situations depends on circumstances like where you are in the 3G coverage, how close you are to a cell site. Things like terrain and buildings all come into play," Siegel said. "I'm not denying that people are having a less than satisfactory experience, but overall, the phone is doing great."

Nomura Securities analyst Richard Windsor said the problems being reported are "typical of an immature chipset and radio protocol stack", and that the blame may lie with Infineon, makers of the 3G chipset.

Windsor warns the problem is unlikely to be resolved via firmware updates, GigaOm reports, meaning replacement may be required.

More damning evidence from T-Mobile in the Netherlands, which warned on the company blog: "The 3G coverage of T-Mobile is as good as the competition... We suspect that it is a hardware / software specific issue of the iPhone itself."