Many fire departments use alphanumeric paging services through third-part commercial paging carriers (such as Metrocall or USA Mobility) to augment their department/municipally owned radio dispatch systems. These systems carry messages not only for fire dispatching, but also from the general public and other businesses (such as doctors, lawyers, etc). They are connected to the public telephone system, and are therefore covered under the Electronic Communications Privacy Act, United States Code, Title 18, Chapter 119. It is illegal to intercept any communication on such a system not intended for you. It is also illegal to manufacture or distribute equipment for this purpose.
Some departments/municipalities have built their own alphanumeric paging systems; where the transmitters are owned by the department or municipality, and pager messages are reserved for fire and EMS dispatch and communications. These systems are not connected to the public telephone switch, and are not used by the general public. It is not illegal to intercept messages from these systems, and would be no different than monitoring the tone/voice pager messages from a fire/EMS dispatch channel. I am not aware of any departments in the Baltimore metro area that own their own alphanumeric pager systems. There are a handful popping up in Southcentral PA.
I hope this clears things up.