Arkady:
Callsigns on strips were introduced as a temporary measure when we went from 2 to 3 letters (eg BA became BAW). When the computer chaps wanted to take them off again, they were prevailed upon to retain this valuable feature.
Occasionally there appears a 3 figure group which has not been encountered before, and has been said, there is almost always time to look it up in the always to hand ICAO doc. (or ask the preceding sector/unit what he`s calling himself).
Trusting ones ears is always a bit perilous, I used to think NEATAX was MEATAXE for years.....no one ever complained.