Modem Problems

I have a pentium 75 with a 56k Diamond Supra Express ISA slot modem on board. When I run the diagnostics program in Windows, Windows tells me there are no problems with the modem. In control panel there are no conflicts with the modem, but when I go to dial an ISP, I get no dial tone and a window pops up that tells me there is no dial tone. The modem came out of another machine that it worked on. Do I need to go to an AGP modem? How can I fix this problem before I go out and buy a modem?