have recently changed routers from Netgear to Vigor to support a VoIP network.

Our distributed office was managed quite successfully via PPTP VPN protocol on the Netgear system.

Since switching to the Vigor2830n router remote access has been denied due to GRE protocol problems.
I've set the router up the same as our Netgear in terms of port mapping. The firewall on the Vigor model is disabled.
I've configured the firewall on our SBS server and remote PCs to allow traffic on those ports.

I am assuming that the problem is at the router incoming end as;
it was working perfectly well prior to changing the router
I don't see any PPPTP traffic on port 1723 in the NAT Sessions Table Diagnostics.

What am I missing?
I really don't know where to go from here apart from back to our Netgear which I don't want to do as the Vigor is supposedly more reliable for the VoIP service. I need to get this up and running ASAP as it is impacting our business.