Shoretel is upto Version 9. The sip support is improving but is still lacking. As for service over the internet it is hit or miss. Depends on your provider. (No QOS on public internet) I do have a couple of remote locations and the service isn't to bad most of the time. 2 DSL locations 1 Cable location.
As for ease of use. Shoretel is the easiest system around to install. I can do a nice size facility in about 2 days. 1 Day setting up the system. Entering Names and 1 day putting the system in place. I use DHCP and POE so that makes it much easier. I even have it using GFI fax server. I have a GFI fax server centralized then each location forwards thier inbound faxes to the GFI software and routes to thier email. All this is done over our MPLS network. I about 12 locations using 2 Voicemail Servers.

It is a great system but it is VOIP which has some draw backs.