If the phone has a dial tone but does not ring, adjust the ringer loudness. The location of this adjustment depends on  the particular phone. In the case of older land-line phones, it is a slider on the bottom of the phone. In the case of newer electronic phones, it is a volume control accessed through the dial pad or menu on the phone--you may need your phone's instruction manual to sort this out.

Also check to see if there is a switch for turning off the ringer, and--if there is-- turn it on.

In some cases, if you connect more than five phones to a given line, they won't ring properly. If this is your situation, you may have to disconnect one or more phones, or at least turn their ringers off to get the ringer working on a particular phone.