Recently I had a VPS customer that was looking to get a GoIP GSM Gateway working with his Asterisk VPS. These little SIP/GSM gateways can be used to connect Asterisk to the GSM/mobile network. The single-sim models can be had for around £100 from e-bay.
They are not the easiest boxes in the world to set up, with the web GUI being rather confusing. Eventually though the GoIP was configured correctly but still calls we intermittent, with connection to the Asterisk server being lost.
As the GoIP box was being hosted behind a residential ADSL router the issues were typical of NAT/Firewall problems. They were probably being caused by a mixture NAT and a SIP ALG (application layer gateway) in the ADSL router.
The solution was to use the new SysAdminMan VPN:PBX template which uses OpenVPN to create a secure VPN tunnel to the Asterisk server. The customer purchased a TP-Link 1043 router, flashed the SysAdminMan firmware, and the GoIP was connected to the VPS over the VPN. This meant that private IP address ranges were used, with no NAT happening at all. This is a great example of how using VPN:PBX to provide a VPN connection to your Asterisk server is easy to deploy and works around any NAT/Firewall issues.
Here is a diagram showing the customers setup –
See here for more information – http://sysadminman.net/sysadminman-vpnpbx-hosting.html