Steve Brack writes in a message to Charles Hunter
 SB> How  did individual units identify themselves to the "system" 
 SB> for  billing purposes, and how did individual phones know when 
 SB> it  was them being called? 
I'm not sure how the old "operator required" system did it (I arrived on the 
scene too late for that) - but the IMTS radios had a "control" board where 
you programmed the unit (phone) number by soldering in diodes to a matrix.  
The radio would send this number to the switch when it went off hook and 
would respond to rings on that number.  If it hadn't been addressed, the 
radio remained "squelched" so it couldn't hear other traffic on the system.  
And besides - it scanned to look for the idle tone on one channel of the 
system.  (I don't recall how many channels the radios could support - but I 
do recall it was at least 4 and that the radios required expensive "channel 
elements" (crystals and oscillators) for transmit and receive.)
  
These high powered (250W on the base!) VHF and UHF phone systems were a major 
cause of intermod in those bands.  (Just imagine the problems caused by a 
number of evenly spaced high power transmitters...)  Good riddance!  There 
are now fewer things to listen to on the scanner, but what is there is 
cleaner.  (The recent laws would have prohibited listening to these channels 
anyway.  It's a good thing these channels were no longer used by phones when 
the laws were enacted, or there would be holes in our VHF and UHF coverage 
too!)
 
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