After a majpr software update, it was short time after iPhone 5 was released, my iphone4 stopped ringing once a call came. Well, I was able to fix that problem by restarting the phone. Although it was just lasting for a couple of weeks, but better than no ring at all.
However, after the recent software update (Feb.2013), which I was hoping will fix this problem, it got worse. Immediately after that update it stoped ringing and a restart doesn't help anymore.
I have read all articles and have tried many solutions. But no result Have to carry the phone all the time with me and taking care of it's vibrations by the time now.
Restore the phone in iTunes, first using a backup, and if needed as a new phone, without using a backup. If restoring as new doesn't help, there is a hardware issue in your phone that needs evaluation by Apple or whoever provides iPhone service in your country,