That's an engineering test code.
Here in Ireland, when I was a kid, if you dialed 17 and hung up, the phone would ring after a few minutes and there would be all sorts of strange clicks followed by :
"Welcome to the automatic line test facility - Testing... testing... testing... Complete!!! - Line number : (your phone number read out) - metallic path: OK. Ring back: OK - Polarity: Normal - Loop: OK.
To continue, authenticate and terminate with a #.
If you picked up and tone dialed 1,2,3,4,5,6,7,8,9,*,0,# the switch would say "Your keypad is functioning correctly - Test complete, please hang up"
Or, if you pulse dialed 1,2,3,4,5,6,7,8,9,0 - "Your dial is functioning correctly, please hang up"
Now if you dial 199000 it just says your phone number.
19800 will tell you which carrier your calls are with for each call category.
The engineering test codes are much better hidden
