Hello Melissa,
Here is an article about troubleshooting the phone being silent. Called iPhone: No ring or sound from built-in speaker it can be found here http://support.apple.com/kb/TS1205.
Also I would recommend looking at the side switch on the phone. Here is an article that outlines the sideswitch:
iPhone Buttons
http://support.apple.com/kb/ta38625
Flip the Ring/Silent switch to change between ring and silent modes.
When set to ring mode, iPhone plays all sounds. When set to silent mode, iPhone doesn't ring or sound any alerts or sound effects (alarms set using Clock do sound). By default, when you get a call, iPhone vibrates whether it's set to ring or silent mode.
If iPhone is set to ring mode, you can silence a call by pressing the Sleep/Wake button or one of the volume buttons once. Press a second time to send the call to voicemail.
Also, its possible you may have Do Not Disturb turned on. Use this article to see if it is turned on or not.
iOS 6: Using Do Not Disturb
http://support.apple.com/kb/HT5463
Take care,
Sterling