I have a managed internal phone system that gives us free internal calls between sites. If anyone is interested it called Centrx .

What I have in my room is a spare centrex line and I am wondering is there any device that I can plug into this that would allow me to use a VoIP phone to call from outside and for this device to connect me via the spare line to any internal number?

I know that Asterisk can do it but I am wondering if there is any small device that could do the same?

just incase the top isn't confusing enough I have made my worst ever Visio drawing as well.