Call Waiting allows you to answer a second call if you are already on the phone and works on all touch-tone phones. When you are on a call you will hear a series of beeps telling you that someone else is trying to get through. If you want to take the second call you can either:
Hang up. Your current call will end and your phone will then ring with the second call.
Press the recall/flash button. Your current call will be put on hold and you will switch to the second call. Press the recall/flash button again to go back to the original call.