They have their own repeater systems, linked together and located in different places around the city. It is very similar to the service your cell phone company provides you and the way your cell phone works.
Their individual radios cannot talk very far by themselves. It is their repeater systems that allow the radios to have such a long range.
These repeater systems are called Trunking Systems.
If your company needs wide area coverage for at least 30 employees and has a budget of at least $50,000 we can design and implement a wide area coverage system for you.