APCO Sets the Standard for Radio Communications for Public Safety in the USA
In the USA, there is a standard. The standard is set by the
Association of Public Safety Communications Official International (APCO). APCO also sets standards for other public safety communications such as the use of digital communications, known as Project 25 (P25).
APCO Project 14 is the standard that sets the use of public safety radio communications codes, such as the use of the "10 code" and a phonetic alphabet.
Although APCO endorsed the international phonetic alphabet, APCO adopted LAPD's phonetic alphabet (Adam, Boy, Charles...) and made it known and adopted by other agencies. In fact, LAPD's phonetic alphabet is know as the APCO phonetic alphabet. Some agencies make slight variations to it on one or two letters; similarly, some agencies use their own slight variation on the APCO 10-codes, as well.
No agency is required to use any particular standard. APCO is just a suggested standard; but APCO projects are widely accepted as "the standard," particularly for its member agencies.
There is also the NATO phonetic alphabet (Alpha, Bravo, Charlie...) - AKA military phonetic alphabet. Some public safety agencies choose to use this standard.