I have an iphone 3gs 32g OS 3.1.3, a couple days ago stop ringing.
I checked the speaker and is working fine. I can hear music, or the ringtones, but the clik of the keyboard and lock sound are gone.
It's not in silence mode, and all the sound settings are on. And thye speaker works fine.
If I'm listening music using earpiece it ring when somebody call.
Please if some body has the same issue and fixed it, let me know how you did it.