You can follow the standard GL.iNet documentation to set up either an OpenVPN or WireGuard server on your router.

VoIP requires very low bandwidth. You may have to be more concerned about latency and jitter.

I presume you currently have a VoIP provider. It may be easier for your friend to use a commercial VPN provider to get past the blocking, so your friend can access the VoIP provider directly. This should provide better performance/reliability and be simpler than setting up/maintaining your own VPN server.

I do not work for and I am not directly associated with GL.iNet

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