Asterisk-Altigen communication

Hey, I’m have a lot of difficulty trying to configure a SIP trunk in FreePBX to connect to an Altigen phone server. Is this even possible? The idea is for the individual extensions on the different PBXs to call over a SIP trunk. what do I put for username and password? Any ideas for configuring on the Altigen side? Any input on this will be very helpful

Here’s an example of doing it the other way around (Altigen registering to Asterisk).