I am exploring installing an asterisk system to replace a Toshiba (dksub/dksue) pbx box + voice mail box in a medical practice with around 45 users, and a T1 line (4 or so fax units).
Since the phone system is mission critical, I envision doing the swap in the following way while keeping the existing system as is:
Once installed, and configured, and tested as far as possible, I would initially hook the T1 into the new box on a weekend and have some staff help test the system. Every user would have two phones for a time.
If it passes that testing, I would allow the users to use the asterisk box for long distance via voip provider, while the old system is used for regular business.
Passing the above tests, during a quiet period, say a Friday afternoon, again plug the T1 into the asterisk, and have the staff try and test it more fully, and give it a load.
Back to the old system, this time giving the asterisk box a test during peak periods for a time…where if it passes that test, I could leave it in place, removing the old system after x weeks of testing.
Please see if you can shoot holes into my thoughts on testing.