You only need to do that if you're stupid frankly, all you need to do is insert dialing rules that if 911 call send the BTN/main number, if for all other calls send station DID - often this is done on the carrier side - so you should only need to place calls from one phone for the whole site. It's wholly impractical to do this as well for any site with more than 20 phones, I had customers with sites that has over 1000 stations on them.
One of the big players had a lovely-insane policy. They'd use your last MSAG-validated[1] address, regardless what new address you had provisioned.
So, user's in Texas, gets an MSAG-validated address, everything's good. They move to Kentucky, change addresses. The new address would have some MSAG issue, and could take days or longer to resolve.
This company thought the best thing to do was to route the call to Texas.
(MSAG is the street addressing system that the PSAPs use. It can be considerably different from the postal address.)
