Foreign IP guard tool is intended to prevent any foreign country IP from accessing your Customer Portal. You can receive a notification email whenever a foreign IP attempts to access your portal and choose to block or authorize this IP.
For SIP/IAX connection security you can check out IP restriction and POP restriction from the customer portal > Main menu > Account settings > Security tab.
I had the same problem last year and it has been a source of troubles.
I was told by the support team to select a server that was near to my location in the other country, and it did the job.
But I have a question on this topic: would using a VPN software on a smartphone solve this problem? i.e., the VPN would use a server in our usual location, and I could still make and receive calls even if I am in another country? Would that work?