If it's all planned and implemented properly there should not be a mess.
911 issues like this are easy to overlook, but not too hard to address.
If people take the time to implement things correctly, the end user (as they should be able to) can pick up any phone, dial 911 and there should be the appropriate emergency personnel at the other end with the correct info given to them to do their job.

Here is a good read:
https://knol.google.com/k/911-where-s-your-emergency#