Jim
If I could offer my 2 cents, the least costly way to go in your case is have every one with a smart phone download the Zello app either from the ITunes or Google play.
Also download the PC app as well.
You can use the free version, I have been playing with for the last 2 weeks, and it works good.
What is nice about you set up a main channel for your watch group and when someone goes on duty per-say they log in to the channel.
Any one with computer can act as your base station as long as they have internet access.
It use your Wi-Fi internet, as well as 2G, 3G, 4G, or LTE cell service. As for the data used in just voice mode it appears to very low usage.
There is slight learning process, however over all I found to cheap way to have 2 way private radio.
The biggest thing I see is when you set up your user name, it will only allow you to logged on 1 device at time.
For giggle, I put it on old cell that is not hooked up to any cell service and worked just fine off WIFI service.
Also any radio transmission you make is recorded by any unit that receive the signal. < of course you can delete as well>
The nice part is of it you can be on line and working with in 2 to 3 minute, with no added costs to anyone.
I hope that helps.