iPhone 3gs not ringing sometimes

[Noumenon]

My phone is not ringing sometimes, like every other time. The screen will light up when it gets the call, and I can answer it, and everything else works fine, ... but will not ring.

I don't have extra ring tones, am not in air-plane mode, and the settings are correct. Again, it's an intermittent problem.

Has anyone else had this problem? Is this a AT&T issue?

 

[Noumenon]

In case anyone searches for a solution to this problem,...

I found in Contacts that the selected ringtones had a .m4p extension. For example "Old Phone.m4p". I reselected the same ringtone and now it appears in Contacts without the file extension,.. i.e. "Old Phone". Now the phone rings correctly.