Usually, when you initiate a dial-up connection, you receive (via the connection) a new default route. You can see this on NT/2000 boxes by called up a command prompt and typing 'route print'. What this means is that all traffic will try to go out through the dial-up connection to get to it's destination.
In order to prevent losing access to the servers on the other side of your 2621 router when you dial-out, you need to add a couple of persistent routes. In other words, you have to specify how to access those networks explicitly.
Assuming NT/2000 :
If your router is 192.168.0.1 and the network on the other side of the router is (say) 10.100.0.0/16, then you need to type into your PC's command prompt:
route add -p 10.100.0.0 mask 255.255.0.0 192.168.0.1
The -p will make the route addition persistent - which means that it will survive a reboot.
Please note that you'd have to type this for every network on the other side of the 2621 and you'd have to do this for every PC that may make a dial-up connection. If you use DHCP, you could specify these routes that way and get it out to your clients far quicker.
Hope that helps.