HP Vectra undetectable modem?!

I own a genuine HP VECTRA Pentium III , Windows NT.

I have this problem in my PC which doesn't detect the modem.I used to access the internet using a cable connection through the network card. I cut my cable subscribtion and I now want to use a modem dial-up connection, but though I did plugg the modem (it is internal) inside the main board of my computer, it realized that there was an intrusion, but did not detect my modem. I changed it's location to another slot in the motherboard, still the same.

What's the problem?

P.S : This internal modem trying to install is a used modem. I got it from my old computer. But it was still functioning ever since I last used it in my previous PC. Why isn't it being DETECTED even? What should I do?

Please help

Thanks