amphibian There is not too much science to it:
1. Log in to ZeroTier website and create a new network.
2. Install the package in the Issabel computer. Something like yum install zerotier should work.
3. Join the network. From the Issabel command line issue a zerotier-cli join xxxxxxx (where xxxxx is your network ID from step #1)
4. Go to the ZeroTier website and authorize that node.
Done. Your Issabel is now on the ZeroTier network.
What you can do now is deploy ZeroTier on your remote computers that will use a softphone to connect. Follow the same steps #2-#4 to authorize the unique remote computers, and then configure the softphones pointing to the ZeroTier network address of Issabel.
The clients will then connect to Issabel using the ZeroTier network.
We deployed this solution and had about 50 remote users connect with Issabel using ZeroTier.
If your more network paranoic, then you can go to the firewall section and only allow softphone traffic through that interface.