I picked this up from the SEC-ARES Google group from Barry KB1PA.
Before clear text was required, in a Multi agency incident, one responder called
dispatch, which was all patched together for interagency coordination, and told dispatch
he was code 101 (which for that agency meant he was going on a 10 minute
“bio” break). One of the other agencies personnel heard code 101 (for that agency
it meant “officer down”), and they all started to respond to the last known location
of the original officer post-haste for support. Fortunately no one was injured (maybe
some pride was hurt).
We should always keep in mind that the object of communications is to communicate. Q-codes (designed for amateur radio Morse code operators), "10" codes, and all but the most universally accepted acronyms are to be avoided in voice communications.
So "What's your QTH?" is not only incorrect (in Morse code communications it's simply QTH?) and out-of-place in voice communications, it communicates no more information than "Where are you?". When dealing with municipal emergency services, Maidenhead Grid Locations (e.g. FN03gx) are of course useless, and latitude/longitude and even descriptions like "200 metres north of the intersection of Bathurst and Aurora Heights Drive" will slow things down.
That last examples is real. There was a motor vehicle accident near my home with (thankfully minor) personal injuries and I ended up calling 911 describing the location relative to the nearest intersection. The 911 operator wouldn't acknowledge the location until I gave her the address. I didn't know it and had find the sign before emergency services were dispatched.
So unless you know them personally, don't assume that the person receiving the message has knowledge of any of the abbreviations or jargon that you've picked up along the way. If you do useit, make sure you use it properly.
The exception would be the phonetic alphabet. To my knowledge in Canada the standard phonetic alphabet is used universally in public service and worldwide in aviation. I've noticed that it has been adopted by customer service agents in many companies as well.