You will need to be in "the same network" as your box or have a route to it, if you "lose all communication", you might want to previously add another ip/network rather than replace the apparently working one while it works. using "ip" the recommended modern network tool:-
ip address add 192.168.1.xxx/24 dev eth0 (where xxx is an unused address and between 1 and 254, you don't need to obfuscate private addresses 
)
would do that until you sort out your own hosts's ip address/network, both should work depending on the network your access machine is in, if there is a route between your control host and the PBX, it gets more complicated to traverse that route, when the new network/routing works then do the /etc/network/interfaces thing and /etc/init.d/networking restart , don't waste your time or money with any commercial solutions as otherwise suggested, they won't ever work on your hardware.