Change the local SIP port in the phone configuration – refer to the phone user manual to see if it’s possible. or
Use a firewall or the firewall features in the customer’s router to block scanning attempts. Check the equipment’s user manual for more information, or allow only incoming SIP traffic from the IP address of the PBX server. or
Configure the phone to allow only calls from the IP address of the PBX server. Again, you will need to check the phone’s manual for this.