CFSRES was originally set up for the CFS pagers, but the other emergency services thought it was a good idea an adopted it later into the piece...
I believe when it was being designed, they were looking for a string of at least 6 characters that wouldn't appear normally in messages...
I'm fairly certain that the pagers could be programmed to trip to Tone 7 for any string of characters, but all pagers would require reprogramming...