The problem being reported is that when either placing or receiving a call, the phone will reboot itself during the call. As one can imagine, a phone rebooting during a call renders the phone useless. The Apple support forum has a 33 page topic discussing the issue where users are complaining about their phones rebooting during calls of varying length.
"After about 4 or 5 mins in a phone call my iPhone 3GS reboots itself for no apparent reason. this did not happen before I put the iOS4 update on it. Is [there] some way I can stop this [from] happening, or revert back to the previous version? "
There are countless reports of this issue across a wide range of geographies and networks. While there are suggestions the phone is overheating from the iOS update or there's a software bug, no one has actually determined a solution to the problem. At the time of writing, Apple has not admitted or acknowledged the bug but that could change after the iPhone related event being held on Friday.
The bigger issue for the European users, outside of the UK, is that the only known fix is to revert your phone back to iOS 3. While this can be done by users in the US or the UK at Apple stores, those in other countries do not have this solution as the phone is sold by third party vendors. Because of this, users are stuck with phones that reboot after a call of a few minutes without a solution.[/img]