Samsung SGH-T459 Gravity - beeping problems

I have a Samsung gravity and when I am talking with somebody on this phone, I get this beeping noise. It is the exact same beep you hear when you have another incoming call or if somebody sends you a text while you are talking - except that nobody is calling and there is no text. This only happens when I am talking to somebody and the beep is not an external noise - that is, it comes through the earpiece of the phone. What is this and how do I make it stop?

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Is it with every call you make?