The ringer is done with the speaker on the phone. Try pressing the Speaker key on the phone (the one beside the up and down arrows) and see if you hear dial tone. If not, then press the up key to see if you can eventually hear dial tone. If you don't hear anything with the speaker, then the speaker is shot.
Alternatively, press the superkey (the blue one) and choose "No" until you see "Ringer Adjust". Choose "Yes". You will see "Adjust Pitch", Choose "No". You will then see "Adjust Volume", choose "Yes". Use the up and down keys to adjust the volume. If you don't hear the ringer with pressing the up key a bunch of times, then the speaker is shot. Replace phone.