The radio identification numbering system was designed and approved by the Souhegan Mutual Fire Aid Association (SMFAA). The SMFAA is a Fire Mutual Aid Compact formed along the watershed of the Souhegan River. It includes the towns within the area roughly bounded by Hudson in the East, New Ipswich in the West, Dunstable Mass. to the South, and Weare to the North. There are 20 towns and 1 city (Nashua) in the association.

The radio identification system involves three variables. The first number is the Town ID. This number is based on an alphabetical town list from Hillsborough County (with a few exceptions). The second part is the type of resource, i.e. Chief Officer, Engine, Ladder, Tanker, etc. And the third part is a number identifying the particular resource within the broader type.

For instance, Mont Vernon Engine 2 has the call sign of "21 Engine 2". That is calculated using the town ID of 21, the type of resource is "Engine", and in particular it is Engine 2. The Mont Vernon Fire Chief is "21C1"; Calculated using the town ID of 21, the type of resource is chief officer "C" and in particular, he is chief officer number 1.

The following tables indicate the words, numbers, and letters used to figure out radio call signs in the SMFAA area.