VPN Issue

Is your LinkSys router doing NAT? If yes, then there's nothing that you can do except replace that router with a router that supports PPTP:

http://support.microsoft.com/default.aspx?scid=kb;en-us;234163

You need to enable Port triggering on the linksys router.

Also are you using IPSec or PPTP?

Anna adds in his comment that you should call Lynksys. You don't have to since most Lynksys Routers will allow you to do port forwarding.
First for trouble shooting:
To make sure that the problem is the firewall, type the following command at a computer that you get the error 721 (open a command prompt by typing cmd in the run box on win2k machines, and command on win9x machines):
telnet serveripaddress 1723
(Where serveripaddress is the public IP address to the VPN Server)
if you get a blank screen the problem is NOT with the router/firewall, if you get can't connect then the problem is with the router/fireall.
On most routers you can use port forwarding which means that you can open your firewalls IP address to receive packets for a specific TCP port number (in your case port # 1723) and forward it to any internal IP address. In your case this is what you have to do, you have to configure your firewall to allow TCP Port 1723 to your VPN Server, and make sure that it gets forwarded to your Server, then you use the IP address of the firewall to connect from clients.
BTW, it is not TCP Port # 47 that you have to open, it is Protocol 47 and TCP port 1723 (TCP is Protocol # 6). In most cheap firewalls you can't configure the protocols, which leaves you just to open TCP port 1723.