I've experienced two instances of the same problem this week; both occurred after rejecting a call while doing something else (texting, listening to music). A simple reboot of the device cleared the problem both times.

Were this a PC-based device, I'd suspect an IRQ conflict with the phone application perhaps causes it to lose resources after rejecting a call that it can't reclaim to make a new call (old-school diagnostic, I know).

Anyone else got ideas on this one?