In Many cases VoIP doesn't always work well with NAT. Connecting a phone through the Internet is not supported. It would be very difficult to get this to work, if it was at all possible. There is a lot of communication between the phone and the Shoretel server and the voice switches. Even if you got trunk calls to work, intercom calling would be very unlikely.

You will need a VPN for the phones to work. They do have phones that can create their own VPN tunnel, but these require a Shoretel VPN concentrator on the system side to work.