My iphone 3GS is not ringing when somebody is calling me. it is only vibrating but not producing any sound to show someone is calling.

If the switch on the side of the phone is flipped down (red showing) then it is set to silent/vibrate mode. If that isn't it, then go to your home screen and try pressing the volume up button on the side of your phone several times until it reaches the desired volume.

Strange. I would try resetting your iPhone. Hold down the home and power buttons until it powers off. It should start up automatically but if not just press the power button. This has worked for me in the past when my iPhone has acted strangely.
Don't worry about a reset or hard reboot. You won't lose any data or settings.

Contact your cell phone service provider on this.

try going to apple site and troubleshoot iphone

make sure the ring/silent switch is not showing an orange dot... it's right above the volume buttons. try to play music see if there is water damage (any pink/red in the charging port or in the headphone jack restore the iPhone in iTunes if that doesn't work you've got a hardware issue

did you check the little button thingy on the left top part of the phone? i discovered that thing, and i was like, "Whoa!"

It has been damaged. Contact Apple for replacement.

simply press the button next to volume control. When you pressed the button you will see red dot. that's mean your phone goes to silience.

Other option is: Go to setting, scroll down choose sounds, and touch Vibrate and ring..

Make sure the switch on the side is not pushed down. You shouldn't be able to see Orange.
Rob

----------------------------- ExecTones http://www.exectones.com/ iPhone Ringtones for Business Professionals

There's a switch on the left side at the top of the iphone that turns the ringer on and off. Make sure this is towards the front of the phone to enable the ringer