We have been unsuccessful using the GXP2130 v1 and v2 phones. We can successfully connect Zoiper and PhonerLite soft clients (as a test case) to FreePBX using SSL and VPN, however GXP2130 does not work in either mode.
We have contacted Grandstream helpdesk and they attempted some troubleshooting, however ultimately the answer was that their phones work with their PBX system and they were unable to get them working with FreePBX. I appreciate their effort, but typical vendor answer. The logging on their phones in this regard is lacking. They claimed some issues with VPN in 220.127.116.11 firmware. I haven’t tried with the latest firmware 18.104.22.168 so I cannot confirm if the problem is still there. It has taken up too many hours for me to try again, maybe in a few weeks I’ll give it another whirl.