P25 is not a band. P25 is a digital voice protocol used in radio communications on various VHF and UHF bands.
The agencies that you volunteer with most likely have a license for their radios. The license holder would have to authorize the use of personal radios under their license. Any radios that you bring to the party would have to be suitable to use under the agency's license. Public safety licenses are generally not issued to individuals.
FCC licensing requirements are not driven by what radio you have. The licensing requirements pertain to the radio service in which you plan to transmit.
FRS does not require a license, but it does require radios that meet the FRS rules. The Motorola XTL2500 does not comply with the rules for FRS because it has a separable antenna and outputs too much RF power.
GMRS requires a license and the rules require that radios used in GMRS meet GMRS rules.
MURS does not require a license, but it does require radios that meet the MURS rules.
Amateur radio requires a license. The amateur radio rules are less specific about the radios used than FRS, GMRS, or MURS, the the amateur radio rules do specify that the radios used meet certain emission standards.
None of the above radio services are suitable for public safety communications.
A better approach would be to determine your communications goals and then find a radio service that will help you meet those goals. Once, you decide which radio service you want to use, then acquire radios that are permissible to operate in that service.