iphone 4s wont ring only vibrates when charging

I am a new iphone 4s user when I plug my phone in at night or charge it, it will not ring it only vibrates if someone calls or texts me it will only vibrate. I have checked the slider button on the side its not stuck on anything like that, the sound settings are fine i have a ringtone for both calls and texts. I have called apple and they suggested to do a back up and restore which i have done that and the phone will ring with a call but not a text so i'm still having the same problem.? My husband has the same problem & we got the phones at the same time, ( we thought it was a setting and we weren't familar with them yet) I have looked online and havent managed to find a resolution to this issue yet. I hate to take the phone back but its frustrating when you miss important texts =( Any Suggestions???