ADSL and PPTP on SBS4.5

I have been trying to get PPTP working on my SBS server for some time now without success. I have updated my cisco675 router to 2.1.0a cbos to enable handling of packet type 47 GREs. I am now receiving an “error 721: Remote PPP peer not responding.” I have enabled nat in the router for port 1723(PPTP port)and still the same result. Does anyone have any thoughts on this problem?