Help annoying tone when using keypad during call

[nodgnoc]

Greetings. During a call I sometimes have to hit a number on the keypad to access the proper extension. Whenever I do this, I get a harsh tone that lasts for about 15 seconds. It's similar to a dial tone but louder and more annoying. I've been through all my phones settings and all sounds have been turned off on the phone during a call. All of them, yet the tone persists. If someone call tell me what this is and help me alleviate it, I would be most thankful. Thanks for your time.