Hi Hovus,
I have never run a phone in teleworker mode with a PS1 but I would have thought you point the phone to the PS1's public IP address but I might be wrong.
With regards to the reasoning for the IP addresses please see below taken from a doc I got from Mitel which explains it briefly...
The PM and the PEC will have a Private and a Public Static IP address
When the phone boots up it will be programmed to look for the External IP (Public) IP address of the 5000 Processor Module (PM).
This will be forwarded to the statically mapped Internal IP address of the 5000 Processor Module (PM) and so the contact is made.
The Processor Module will reply to the Phone using the Route it came in on and supply the details for boot up.
When a Call comes in it will know to send it down that route as there will be a relationship now between the PM and the Phone.
On call setup - if there is a resource problem, or compression is required, then it may include information in the packet for the phone to use the PEC for the speech call, however data will always communicate with the PM.
The external address for the PM and the PEC are programmed in the P6xxx numbers in System/IP Connections/P6xxx/NAT IP Address of DB programming.