Someone posted the proposal for the system from back in 2018 here. I'm guessing they have the other agencies programmed into the radios:
By utilizing a system that is interoperable, Augusta will be able to continue communicating with other agencies during disasters and on a daily basis, as needed. By providing this level of communication, Augusta will be able to minimize the loss of property and the loss of life during a disaster. By purchasing our own radio system Augusta will have the ability to connect to surrounding agencies and communicate seamlessly during disasters and for the purpose of day to day communications. Agencies within the coverage of Augusta's system would also use the system and possibly provide a method of recovering some of the capital required to build the system and assist with the ongoing operating expenses.