There have been cell phone apps which are integrated into land mobile radio systems for several years. With the advent of FirstNet I suspect the interface between large area trunked and local networks will be more common where users have access to LMR using smart phone or tablets.
Here is a link to a app based solution to where cell phone can interface with a two way radio based communication system whether simplex, duplex or trunked.
Using the JPS VIA push-to-talk application, users can communicate via Smartphones and tablets with associated radio systems.
www.jpsinterop.com
On the JPS app a cell phone user can communicate directly with other app users without interconnecting with the radio system. The communication between smart phone users basically is voice over IP just like Nextel. If the app user selected to communicate over the two radio system the transmission is basically patched into the two way radio system. Unless the radio system is encrypted the cell/smart phone transmission should be received by any radio on the network.
The advantage of the JPS network is an app user could have communication to the Agency radio system from any location as long as the app user had cell or internet connection. A Chief of Police could be in Florida on vacation and start monitoring or communicating with his Agency in the midwest via the app.
Another advantage is an app user can have radio access from his cell phone if he comes upon an emergency where his assistance can be beneficial .