‣Turn off the silent ringer switch.

‣Make sure you set the volume to approximately 75% percent.
At higher volumes the built-in speaker will distort the sound, rendering it unrecognizable to the phone.

‣Not all modern phones will recognize DTMF tones anymore.
If your phone’s user manual doesn’t specifically mention DTMF support, you can try your phone's compatibility by using a free online DTMF tone generator like this one.