Since the availability of chan_dongle for Asterisk - an open source software implementation of telephone PBX the chance raised to route GSM calls like telephone operators do.
The device above (based on a Raspberry Pi) receives GSM calls via a cheap UMTS modem feeds them into a small Asterisk installation (Site B) which than routes the call according to your setup to the target PBX (Site A) through an OpenVPN connection. Established peer connections are show on the yellow LED and driven by Asterisk event management.
In order to handle power management and prevent sudden power loss the device uses a power management board made by AndiceLabs (Power indication by left green LED). The device checks for power losses and gracefully shuts down once power missing was triggered. Bootup happens again once power is available again.