[asterisk-users] High Availability Asterisk PBX

[RESEARCH]

Hi 
I have the following scenario
A. A PBX on location A with network 192.168.1.1 with extension range 1XXX
and connected to the PSTN Network via the E1
B. Another PBX on location B with network 172.30.18.1 with extension range
2XXX and connected to the PSTN Network via the E1

I need to configure the system and the endpoints such that when one system,
says, A goes down, the system B assumes A responsibility. HALinux would have
been my answer but this should work only on the same subnet

Any advice
Sam