Linksys PAP2 & D-Link DSL-504T Router

Hello,
I'm having trouble getting my PAP2 to work through a D-Link DSL-504T router.
I know I have the PAP2 setup and working because I have tried it at home on a Linksys WAG345G wireless router.

I have opened up all the ports in the D-Link to forward to the PAP2 eg. 5060 etc.. but still no joy.

I don't understand why it worked with my Linksys without doing anything. Does the PAP2 talk to the Linksys router and tell it to open the ports.

Any ideas anyone before I go and by a Linksys router for work.

Thanks

Ben