Default [Q] Key pad not registering during phone calls

My Sprint Galaxy S5 is not registering my key pad presses during phone calls, ie to select menu options. I did an online chat with a Samsung representative who recommended that I clear data and cache from Phone and Contacts app and do a soft reset but it is still not working. I do not hear a tone from my end, and none is being received by the phone menu. I toggled a variety of settings in my phone app (including "long DTMF") and even downloaded a 3rd party app (ToneDef) to send a DTMF signal but none is working. I would appreciate any insight.