I am recommending the UCx from E-MetroTel, it's built on the legacy of Nortel by Nortel engineers and developers. Many customers of theirs already use this product for a hosted solution. One of the great benefits, is that you can mix so many different phone types, especially if any of your customers currently have Nortel phones, which they can continue to use and mix with new solutions. The licensing structure allows you to have 1 license per extension, which includes remote user. You do not have to purchase a remote user license. You can also initiate find me follow me with your devices.
What makes the UCx easy for a VOIP solution with multiple locations is that you can easily place remote IP clients on this system without VPN's and other extensive networking. It's quite easy to integrate an IPhone or Android smartphone app for a soft phone client as well. I currently use Media-5-fone app phone on my iPhone, and I can access system features such as paging, voicemail, transfer, conference, hold music, etc, even if I am 3,000 miles away from the host system....pending you have a good cellular or wifi connection. You can lose the desk phones if that's your preference, or have a mix of desk phones and soft clients and iPhone users, etc.
You can set up a large attendant console with a SIP phone with KEM modules and monitor the lines that way.
You can possibly attempt the same setup with a remote hosted company such as 8x8 or Ring Central, but then it's very hard to control what is going on with your own equipment, you have to be dependent on them.
I am sure the Shoretel product is reliable, it's just that they are so proprietary in everything you do.
Avaya IP Office would be another good solution, I am not sure how their remote licensing structure works, but it's built on the legacy of the Bell system.
If you only needed 3-5 SIP phones, the least expensive option would be going with buying your own SIP phones and setting up a SIP account from VOIP.MS. It's not recommending for a large SIP implementation.
Take a look at all the options first, don't just jump right in, the water might be too cold!
Joseph Sus Jr. Nortel Enthusiast