Who I have the same questioned this topic

Hello everyone.  Whenever I am in a call with my mother on either her home phone or cell phone, I hear a single beep noise on my end, as though someone is pushing a button on a touch tone phone, and I hear it more than once.  It is not an alert for any new text messages or voice mails.  I do not have a smartphone at this time.  I currently have AT&T Wireless and I have already posted this question on their forums.  Does anyone know what could be causing this beeping noise?  It also happens when I am talking with my grandmother, who I call once a week.