X-XXXX is the last 5 digits of your phone number. Doesn't work with 879 numbers. I made a recording of what it sounds like:https://soundcloud.com/jack-rosenthal/99x-xxxx-ringback-steamboat
Also some other cool numbers to check out:
959-1000 Milliwatt, Cuts off after 5 seconds to a silent line.
959-1020 Milliwatt, Stays on sometimes, other times it cuts off to a silent line.
959-1040 Odd high pitched tone (roughly 1200hz), cuts off after a few seconds
959-1050 Same high pitched tone, but flashing the hook gives you a stuttered dial tone. Unlike the regular dial tone, this one gives you a quick burst of dial tone after the first digit has been dialed.
Most other 959 numbers bring you to a silent line, with a slight hum in the background.
The switching equipment is an AXE10 and one of the things I have noticed unique about it is how it behaves when the other party hangs up and you stay on the line. You will hear a click-buzz-click sound, followed by roughly 30 seconds of silence, followed by another click-buzz-click, two rings, then "If you would like to place a call..." followed by a howler. If you stay on the line while the howler plays, it will cut off after like 3 minutes or so, click-buzz-click, and then you can dial a 10 digit number during the silence, it will complete the call for you.
A very interesting central office. Not sure if this happens in other mountain bell areas or not.