Yes, An addon type approach may just work. What could be done is to build the thing so that all the lines go through the add on controller. The addon conroller can be setup to monitor each line for its secret code and then when it is recognized the new uC can send a "0" down to the existing system to request the outside line. The existing system can then be pre-set so that all the lines are configured for "0" operation. The tricky part of this approach would be on dealing with how to provide the connectivity of the phone lines through the new controller. Most likely the new controller would need to provide the connections each through a relay. When the local lines are not passed through then the new controller will have to supply loop current and dial tone capabilities for each line.

Now for the reality measurement. Please approach this project with great caution. It has all the markings of a project proposal made by some managers that do not have any concept of how much work this could turn out to be. You could get yourself buried in it only to find that they will be displeased when they see how much work is involved. And of course from the managers view point it is the money cost that this work translates to.

If I were approached to do such project I would first try to get them to go back to the EPABX manufacturer and tell them that since they wont support the product any more then please hand over the software source code. Then I would prepare a cost estimate to either modify the existing source code, an estimate of of the cost to produce the add-on controller, and contrast those costs with acquiring a different phone key system that has more of the features that they want. This sequence of events itself is quite a task and as such you should offer to prepare it for them as a study that you would get paid for. This way you can make money whilest preparing the proposal !!

Michael Karas

PS Note that there is quite an active market for used phone systems. They should be able to find a suitable used system at substantially lower cost than a new one.
MJK