At first I was going to suggest that it was one that you had to soak in salt and water overnight..... But I gather that they are just old-fashioned phones that can't do what modern ones can - so it's just another 'layer' of censorship really:-
"The “kosher” phone carries a stamp signifying its approval by rabbinical authorities, who will recommend haredi community members to take out contracts with MIRS Communication.
"The Motorola handsets have been modified to disable internet access, SMS text services and video and voice-mail applications.
"Growing concern that new-generation mobile phones could threaten the conservative, ultra-Orthodox values and way of life, particularly among the young, led to a rabbinical committee being formed to examine the issue."
I don't know, probably something similar to kosher washing detergent. From what I have heard, its a sales gimmick. The kosher logo is a trademark and the user company has to pay for its use and since it helps sales with people of Jewish faith, companies choose have to 'certify' their product kosher. Feel free to correct me.