Interrupted Calls

One of our phone lines has begun to have a problem. If you are on the line talking to someone sometimes you will hear a clicking sount like call waiting although we do not have this feature on this line.  If you try and click over like you would if you had call waiting it hangs up on the person you were talking with.  If you simply do nothing the line does come back to the person you were talking with but you miss part of what they were saying because the other person don't hear any sound on their end of the call.  Can you please help resolve this issue, any suggestions?