One famous kind of test number belongs to NYNEX, the regional Bell
> telephone company operating in the northeast U.S.A.. In New York at
> least, there are "9901" numbers, or local numbers of the form xxx.9901,
> which result in a recording which identifies the exchange represented
> by the first three digits. The 9901 numbers may not necessarily exist
> for all combinations of first three local number (central office code) > digits.
>
> All these tests and services vary with each phone company; they are
> not usually found in the phone book, needless to say.
In the old New England Telephone days, I used to dial 9816 for a ringback. Not sure if the "6" could be any digit or not..
--Gene