As I noted above, different areas do things differently. Around our community, we have a fire tone and an EMS tone for each station. (On the fire tones, Tone B is lower in frequency than Tone A - on EMS tones, Tone B is higher than Tone A. That way if you are listening to an open channel you'll know whether a fire or EMS call is going out).
The main purpose in two different tones is this. On EMS calls, only the on-duty crew is to respond. There is no reason to wake up the volunteers and off duty career folks at 3 AM about an incident to which they won't respond.
We do have an All Call tone that alerts the 600 plus firefighters in our county. That's used most often to let us know of severe weather impacting the area. Before, we set off all tones. There are some 30+ tones and at 4 seconds per tone group, that took a couple of minutes. Most pagers re-set themselves after a period of time. We were having problems with firefighters hearing weather alerts because their pagers re-set before the alerting process was finished.
Our county also has a couple of specialized operations teams. Haz Mat, Water Rescue, Confined Space and Trench Rescue. This All Call tone is used to alert them to a call.
The All Call tone is a long single tone.
We also have an "All Chiefs" tone. That way if there is a need to alert all chief officers about some important administrative information, we don't have to go through all the tone groups.