(Guide courtesy of VE2REH)
https://ve2reh.com/wp/hamshack-hotline/installation-et-configuration-grandstream-gxp1625/)

NOTE: This guide is for configuring the Grandstream GXP1625 for use on the HHX Experimental Server.
For HHUX Experimental Server users, please see notes at the bottom of this page.

Once you get your phone, you will need to open a new ticket and select
HHX New Line Request.

(Use “HHX” [Experimental Network] as this endpoint is currently unsupported)

The HHOPS helpdesk can be reached at: https://apps.hamshackhotline.com:9090

Once you have been emailed your provisioning credentials by the HHOPS team, you will need to manually configure your phone to operate on the HHX Experimental Server.

Below are the settings to use to configure your phone.

HHUX Experimental Server Notes:

Please understand that there are some slight differences in the configuration setup if you choose to use this phone on the HHUX Experimental server.

First, the SIP SERVER mentioned above is strictly for HHX. If you use the HHUX server, your email will contain a different SIP SERVER.

As this phone can only be used on our Experimental servers, we are not able to help you troubleshoot any difficulties you may experience in getting your phone operational.

This guide is supplied as a courtesy only.