Term Serves: Works on Dial-up, Not DSL.

I recently tried to access my organization’s Win 2k servers via terminal services. It worked using an dial-up ISP and the MS PPTP client. But it didn’t work using DSL and the MS PPTP client. While troubleshooting the problem, I performed the sametest using a notebook PC with the same results in both cases. If it matters, I’m using a DSL connection with a dynamically assigned IP address. What am I missing? TIA
Tom