Error 20 Subscriber absent only on incoming calls Grandstream BT200

I wish to configure another phone GrandStream Budge Tone-200 that is outside local network.
With other phones I’ve had no problems. This one, register, I can call from it, but it doesn’t receive calls with error Subscriber absent cause 20. I contact my freepbx server not through SIP default port 5060 but with another that is specified on phone settings. This port then, is redirected to 5060 on local network, by modem/router. What can I do?
I have checked NAT that is =YES.
On the phone I have no idea on what to change.
I also updated the firmware.
What can I look to?