Here is what is listed about any phone number that has appeared on the Simpsons. It is straight from the best reference that I know on the web - www.snpp.com!
According to Principal Skinner's rolodex card at Springfield Elementary:
Before Simpson and Delilah After
Home: 555-6528 555-6832
Work: 555-7334 555-6754
In Season Three, their home number was 555-8707.
And the phone numbers from Homer's television debut in Mr. Plow are:
Home: KL5-3223
Business: KL5-3226
In The Canine Mutiny, Bart fills out a credit card application form with the phone number 555-3126. And in Dog of Death, the phone number on the lost dog poster is listed as KL5-3457.
The letters "KL" are in reference to the outdated practice of using letters in telephone prefixes, which was a common practice up to the 1960s. "Klondike" is a code name given to the letters to use as a mnemonic device. The song "Pennsylvania 6-5000" and the movie "Butterfield-8" are both named for telephone exchanges (PE6, or 736, and BU8, or 288, respectively). Other examples are NEptune and SYcamore.
Anyone with an alphanumeric phone can easily see that KL5 is the same as 555, which is the stereotypical phone prefix used on just about every television show ever produced.