You could do it with an internal modem on one of your PCs and a gateway software running. However, the more convenient way (especially considering the future switch to a cable broadband connection), would be to get a router with a serial port onboard, that can be used with external modem to dial up to the ISP. In the future, you could use this setup as a fall-back for cable. Should it drop for any reason, the router would dial the ISP instead.
There are number of such routers on the market, some are designed specifically for the residential applications. Just look for the specs, make sure a serial port is available in addition to Ethernet for WAN connection.
Dmitri Abaimov, RCDD