Ringotel Shell mitigates security risks by communicating with your PBX only form the trusted networks. Depending on the selected region, please add firewall rules to allow incoming connections from these IP addresses:
22.214.171.124 (US West)
126.96.36.199 (US East)
Allow incoming access from the above IP addresses to the SIP (TCP/UDP) and RTP (UDP) ports configured on your PBX.
This list of IP addresses and regions is not exhaustive and will be occasionally updated.