Sami,
what kind of solution are you looking for ?
Is the concern the connection from ATM to central location ?
Or is it a concern about the server at the central location ?
For connection issues, I don't see any other solution than providing multiple lines.
If the concern is the single server, you could have a loadbalancer somewhere in your network.
The loadbalancer can use probes to check the health of the server.
If the primary fails, or is unreachable, you can automatically redirect the traffic to the standby.
ATM machines will point to the virtual ip (you could reuse the current ip and assign a new one to the servers).
Not sure where to place the loadbalancer without a better idea of the problem to solve and the network.
Gilles.