I have a closed Ticket #202117910
In this case, the SIP Trunk registration issue was resolved after the country IP restriction turned disabled.
The ITSP's trunk IP is 220.127.116.11, when I check geolocation it seems allocated to our country.
Strangely, the SIP Register request is not sending the password information inside the packets while the allowed country settings enable. I think something is wrong with the country's IP restriction mechanism.
For your information.