Not working incoming calls Error Subscriber absent cause 20 on Grandstream Budge Tone-200

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 look to?