In answer to your questions.
1) No they do not use the same voice mail system.
2) Yes both phones are 5360 IP
3) No they have phones on each side that are programmed with the extension for that side. That is causing issues as when people call the extension in each location they only reach the phone in that location. This question did make me think though that we could set up phantom extensions for two phones in one location. One phone would go to each location and then they would hotdesk in. Although this would mean 911 wasn't accurate, it seems like in theory this would work and be a simple extension.
4) Dynamic Express Extension is an option and if nothing else works that may be our option.
5) Our installer tells us that we do not, but wasn't sure what I was talking about.
1. With disassociated voice mails systems figuring out which side they are getting it from could be an issue; also they would have two separate voice mail boxes that they have to keep up with. Do you have the ability to forward messages between voice mail boxes on each site?
2. That is good so their keymap should look the same.
3. Separated phones is fine especially for redundancy purposes in case one side goes down the other phone will remain working. Also if the network connection is dropped between sites the phones stay up and working.
4. DEE works fine if one site is their main site. You should make sure that the voice mail box it goes to is the one at the main site. Have the secondary phone receive the message indications for that voice mail box. Keep in mind that unless you paid for it they can't send messages between voice mail systems.
5. No worries it would just open up a few options, but it isn't really needed to solve the major issue.
So, after going over everything I would just setup Dynamic Extension Express with their main office phone if they have one. If not than just pick one that makes the most sense and eliminate the mailbox off the secondary one. Make sure not to give them a Dynamic Extension Express key on their phone, but give them a handoff key if they also are tying their cell phone to it. Don't forget to associate the secondary phone so that you receive the message indicator when a voice mail is left in the main phones mailbox.
Thanks,
TE