Here is the "official" tech bulletin on what to port forward for IP phones remotely, slightly dated but should still be applicable.
https://dl.dropbox.com/u/95033600/Intertel%205000%20NAT%20Port%20Forwarding.pdf there are a wide variety of ports/ranges you need to open up and map to the 5000's internal IP address.
Remember you need to set the public IP address in the 5000 in TWO places, first in System-IP Settings-System NAT IP Address and secondly in Settings-Devices & Feature Codes-IP Connections-Pxxxx-NAT IP Address. The only other thing you will have to do is go into each endpoint that is remote and under IP Settings of each individual endpoint change the NAT Address Type from Native (or Auto) to NAT. Also, although this won't effect phones connecting, if bandwidth is an issue you might want to use the Remote Call Configuration and apply it to the remote endpoints, basically all this does is force the phones to use G.729 instead of G.711, lowering their bandwidth requirements, and quality slightly, for audio.
Once those are in place, factory default the phone itself, make sure it is on a network that will provide it DHCP and power up and put the phone in Teleworker Mode to the public IP address with the port fowarding to the 5000.
That is it, there is no special magic or trick to make it work... I have connected literally hundreds of phones, both Mitel IP phones and the original Inter-Tel IP endpoints, in this way and had no issues for the most part. Occasionally I have run across routers (or configurations in routers) on the remote end that are too locked down to allow the phones to connect, but this is extemely rare and can usually be corrected by letting the network administrator know what port ranges you are using.
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