Modem doesn't detect telephone ring

Hello!
I want to use hyperterminal in Windows 98 on two PCs on a local telephone network(wich is not connected to the global network).I tested different computers and different modems, but in all cases ,modem doesn't detect the telephone ring when another computer calls it.But I could dial a number from one computer and on the other end hear the ring voice from a telephone.
I used softwares other than hyperterminal but also they couldn't detect the telephone ring.
Would you please help me?.
Do you think that the frequency or voltage of the telephone network that
we are using, is not standard?.But we call eachothers using telephones.

Best regards,
Arash.