I agree with n2mdk. Most likely it is some type of ANI.
Wisconsin State Patrol uses ANI on most of their mobile radios, however it comes over before the transmission, not after, and sounds like a type of squelch sound before the transmission starts (I can sort of duplicate it with my voice, but I'd have no idea how to spell it!). As a former dispatcher for WSP, I can say it is an extremely useful system, especially when radio traffic gets busy. Any time an officer keys their mike, their unit number automatically displays on the dispatcher's computer screen, in chronological order of when each unit keyed up. It makes it much easier for the dispatcher to keep up with each officer's traffic and not forget to acknowledge anyone. I could go more into depth about it, but I won't here.
Hopes this helps, although I know I babbled a bit.
Don