smash14,
I am not really sure if I should be replying to this post or not, but since no one else has I will give you the standard boilerplate answers I give anyone asking these sorts of questions.
1. Does the Mitel 5000 support VPN connectivity and if so, what is quality like compared to our current MPLS setup? No VPN is not supported by Mitel, but I have successfully implemented it; although if there are issues there is very little in the way of support. When everything is optimal you probably wouldn't notice a difference in QoS or QoE, but when it is not both of those can drop quickly to the point of not being usable.
2. Is there a recommend network setup for use over VPN and if so, what is that? Although it is not supported the best practice is to seperate voice and data into seperate vlans where voice has priority going out the VPN tunnel; once it hits that tunnel though you have no control over what happens which is why it isn't a supported setup. The best solution would most likely to be the installation of seperate Mitel 5000s at each location and then have those talk to each other via the VPN. That way if the VPN goes down at least the location will stay up and working, but trunking will need to be sorted out as well.
3. Is there anything special that needs to be done to the phone to allow them to contact the PBX? They would need to know the IP Address of the phone system if it isn't one large flat network; which it probably won't be. The good point is that you shouldn't have to set the phone system up to use NAT, unless that is already setup for teleworkers already. If you go with the seperate phone system solution then the phones would connect to the local site pbx similar to how they do now with the MPLS.
4. What type of bandwidth per phone should be expected? That would depend on the Codec in use, but even at G.711 when in use it should only be around 115-125 kbps
5. Should we place the PBX in a DMZ and host phones that way? Definitely not, but maybe a MBG at the DMZ which would make all the remote phones use the Internet to access the MBG in order to get to the phone system; your VPN would be a better idea than that since it is more secure due to the encryption you can put on the VPN; although that increases the size of the packets and thus increases the amount of bandwidth each call would take as well.
The issue is that you want to get rid of the best solution for your situation and go to a less favorable one and expect the same or similar QoS and QoE. The problem is even though it can be done there is a higher chance that there will be problems at some point and your vendor will not have good luck getting support from Mitel nor will they have the metrics available to them that MPLS provides, along with its QoS support, to help with troubleshooting or alleviating the problem.
Thanks,
TE