12.3. Timing Trunk Transitions
It's important to schedule any changes at the PSTN connect point in a way that minimizes possible negative impacts on production voice systems. These changes might be rewiring of a demarc to add additional capacity, or moving of a connect point from one location to another, from one floor to another, or from an old PBX to a new one. Usually, major changes like this coincide with the adoption (or move) of a new PBX. Since they can add to the successor create the failureof such a project, their timing is critical. Timing is especially important when "new" PSTN connections are displacing the old onesthat is, if you're replacing an old PRI with a new one in a different building or if you're consolidating a bunch of POTS trunks into a new PRI.
Major changes to trunk connections are often called switchovers . Once you've designed your VoIP topology, and you know the proposed location of every phone, every media gateway, and every PSTN connect point, you've got to map out a switchover plan. This is a schedule for your trunk moves from one PBX system to another, from one voice mail system to another, from one PSTN connect point to another, or possibly all of these.
If possible, coordinate the switchover with the phone company so that any major failures can be discovered immediately and the CO reverted back to its prior state until the switchover can be attempted again. (Third-party implementers of PBX systems are experienced at managing interaction with the phone company.)
Plan to do your switchover during off-hoursnot an hour before the start of business on a Monday. Allow plenty of time for testing of the internal dial-plan after the switchover is done, too. Even if your shiny new PRIs are working right, there may be flaws in your call-flow logic, DID signaling, or inbound hunt groups that need to be worked out before users can start making and receiving calls.