Well, for one thing, it appears that Tiscali is a DSL provider, not a cable provider, so you're in the wrong newsgroup.
But either way, whether it's DSL or cable, your modem needs to be compatible with their system. If they say that only their modems are compatible, then that's the final word. You can't make them change their network or policies just for you. You're not talking about dial-up. Both DSL and cable are essentially big LAN's, and what equipment is connected directly to the LAN is up to the network administrators.
To get a better answer, I would suggest going to a DSL group. If you get cable, come back here.