Per my last message, I am not an athority on FCC regulations, but I have researched them a fair bit. If anyone has anything to add or correct in the following post, please do.
Unless you have authorization from your agency and are added onto their license, or an FCC license of your own, you may not transmit on a licensed frequency.
Generally for a fire or ems agency, the license(s) will be issued to the organization, and a number of radios of different power levels will be listed on the license. You might see 10 mobile radios at 100W and 20 handheld radios at 5W listed, or something similar. They will also usually include a distance from a certain point, like the address of the license holder (agency station in most cases) that the radios can be used. This is for frequency coordination with other radio users. The FCC will only allow two groups to use the same frequency if they are a certain distance away from each other, so that the two systems don't interfere with each other. Thats why there are geographic restrictions on the license.
or an FCC license of your own
The FCC would not issue a second license on the same frequency and location as an existing one (freq coordination again), so this wouldn't work. To get your own commercial license, you actually have to pay the FCC to do a frequency coordination study of the licenses in your area to ensure that you don't interfere with them.
The good thing is that, in the case described above, individuals do not need to be added to the license, and new radios also do not if they meet the type and power listed on the license already. The number of each type of radio refers to the maximum number of those type of radios on the air at any given time, not the number that are sitting around the station. As long as the group does not go over that number, they are still within the license parameters.
The short answer is, check the FCC license for any special or odd conditions, if you are operating as a member of the agency with the license and are not violating the terms of the license, then there should be no problems with legality.
Oh, and modified amateur radios are illegal to use on commercial frequencies because they are not FCC type accepted for non amateur use. However you can use commercial radios on amateur frequencies if properly licensed. Go figure.
