what do the numbers on the call boxes in California mean ?

Question

theres 2 numbers then a space then three numbers example 91 135

Answer 1

If you tell the dispatcher the numbers, the police can locate you.

Answer 2

The first two numbers are the hiway you are on, the other numbers identify the call box so they know where you are when you call. In your example you would be on the 91 freeway at call box 135

Answer 3

One of the numbers might be a milepost. Freeways have mileposts, though California hides them pretty well. You see how the freeways are all getting exit numbers, the exit numbers correspond to the mileposts.