I note that before any installation work PBX telephone operator must notify, in whose exchanges are you going to join. Otherwise, you may fall under certain penalties. Because now your subscriber line status change and will act as a connecting line between telephone exchanges with a greater load, and, of course, on the other tariffs. If the local municipal PBX analog, the offender may not notice right away, if digital is usually the case quickly and efficiently. Discovery Communications might disagree with that approach. You should know – that telephone operators so it is better not to joke, if you cause any interruption to other network users, the consequences are serious. Small PBX, included in the 'former' subscriber line may serve its customers in office in accordance with laid down in its service program, and access to it from a public telephone network will be very modest. Several possible to increase the load by asking telephone operators to allocate you a serial number that will cost you dearly, but your access to the outside world will be slightly higher. Elephant, however, does not slide the needle through the eye.
Therefore it is better to have several lines to connect a PBX, and then it will be easier on you to call the 'city' and vice versa. Having two phone lines, can be successfully included in the four-PBX circuit with the connection lines for each direction calls. But in this case should be selected your desired amount of numbering capacity to exchange and resolved technical issues such connection.