1 to 3 digits.
http://www.scils.rutgers.edu/~dalbello/FLVA/infrastructure/infoinfra/telephone/index.html#directory
Here is a copy of the 1900 Madison, Wisconsin, phone directory.
Christian Frautschi's number is listed along with his occupation.
Callers would asked for Madison and then number 315.
http://www.scils.rutgers.edu/~dalbello/FLVA/infrastructure/infoinfra/telephone/frautschi.jpg
zoom in shot:
http://www.scils.rutgers.edu/~dalbello/FLVA/infrastructure/infoinfra/telephone/closer.gif