In the opening credits, Bob answers the phone. None of the buttons light up when the phone rings. On that style of phone, the light would indicate which line to answer. The phone is obviously a prop.
While Bob Newhart is a father himself, he was adamant about Bob Hartley not having any children. When the writers told him they wanted to introduce a child in season six, he told them the script was good, and then asked them who was going to play Bob.