kosher cell phone numbers are like a 'family share' plan and usually it's very cheap to call anyone else that also has kosher phone.
kosher phone numbers are unique and although they can be from most companies so can start w/ 052 054 058 etc, it's the next 2 digits after the 050 that will tell you if it's kosher or not.
kosher numbers do not allow texting or internet and the phones dont have a camera (although you can move the sim into another phone that has a cam, you still cant sms)
you can get a kosher number sim from
www.golantelecom.co.il for 10ils a month
it is not uncommon to see a sign on a bulletin board or advertisement that has a not kosher number and next to it someone stamped on it a stamp that says "notice that this number is not a kosherphone"
i've heard that you can port your kosher number to golan and they will let u have internet/sms.
some schools do pressure the parent body to have kosher numbers claiming it'd be bad chinuch if a kids knows that everyone in the class has kosher phones but my father etc. also some newspapers wont accept ads that have a nonkosher cell number.