How to stop bots calling

Incredible PBX 12.0.70
Asterisk (Ver. 13.4.0)

The main office network is fully setup up and running and the PBX is hosted there and is getting its public IP - I’ve got nothing to mess around at that location.
The remote phone is on the remote network and that is where I am making these configuration. The only connection between both locations is that the remote phone SIP account is configured with that PBX IP from the main office and of course it has its own internal network IP taken from the router. The reason is - if someone call our main office and choose that remote particular extension - we want to dial at the remote location where the employee is located.

Here is a network diagram on the remote location, please note that there weren’t any firewall, we just got that to cut the bot calls.

Port 5060 is open on the main office firewall where is the PBX. Then the remote phone is configured with that PBX IP xxxx.xxxx.xxxx.xxxx:5060 and the phone is working properly inbound and outbound, just receive many dead calls which I don’t know how to stop.
No other remote phones this is the only one.