Welcome to the club, this is a really common problem with the teleworker solution, of which we have yet to resolve. The main thing is the setup of your firewall. Mitel Teleworker does not support 2 port firewalls, eg LAN and WAN, you need a third for a DMZ. You can then NAT through to the DMZ. MitelMatt has reported setting up teleworker and has a good sheet specifically for the PIX, but has a good description of the theory behind the setup.
If you get it working let me know, I'd love to know how you did it.