Googles Nexus S has problems with calls longer than 3 minutes. The handset will reboot itself. Some users report rebooting issues in other situations as well.
Google acknowledged that the Nexus S has some issues, and is working on a solution. Rumour had that the problems surfaced for the first time after a recent update of the Nexus S, that was meant to fix some other bugs.
It's annoying that such a big bug appears in a new handset, but the fact that it was caused by a software update makes that this is probably not a hardware issue, so owners of the phone don't need to go back to the store. We expect an update to come which will solve the problem.
Source: Phonearena.com