Up to you as to how you want to label your units.
Me i use a numbering system for all of my PD units and it is based on the County that they are in. In Minnesota there are 87 Counties and the County is given a number based on its alphabetical name. So in MNs case Aitkin County is 01. So my PD units will be labeled 01-xxxx. If its a specialized unit like a Police Super/Sherriff i add something at the end of the call sign like SGT for sergeant, K-9 for k9, JV for the Prisoner transport.
For Fire it usually an abbreviation of the fire dept and then a Letter and number and the number corresponds with the type of unit.
In my example i will use the Cambridge Fire Dept in the area i am in.
Type 1 i consider an Engine - CFE11
Type 2 i consider a grass rig - CFG21
Rescue Engine - CFRE31
Heavy Rescue CFHR51
Platform truck, or Ladder CFPT61
Water Taker CFWT65
Mobile Air CFMA71
Battalion Chief CFC1
As i stated Callsigns are really up to you, you can come up with your own system.