Umm... these are Teleworker phones, meaning you have a Mitel Border Gateway server?
You should not have to "open" any unusual ports on the far end, the defaults of most firewall/routers work fine. Choppy audio is a not a sign of incorrect port configuration, but of a network issue, most commonly jitter rather than bandwidth.
Get the Teleworker Network Analyzer software off of the MBG, and run it from the remote location, it will identify issues in the ports, bandwidth, and some network issues.