"Do you have your cellphone?"
Translation:Cep telefonun sende mi?
Yes, I think the TIPS on the locative case go over that. Those wondering about var can check out AlexinNotTurkey's post near the top. That other sentence, whose meaning would be different than what we're dealing with here, would be "Cep telefonun var mı?" (and not "... mısın?"), if I'm not mistaken.