You work at a kiosk, which means nearly every person you talk to wants to buy a phone. Otherwise, they would have walked right on by and not stopped. So all you have to do is find out what is holding them back, and overcome their objections.
What do they want? Lowest monthly cost? Most minutes? Coolest phone? Most features? Free phone? Pay no more than $30/month? Widest coverage area? Prepaid plan? No roaming charge? Find out what they want in a cell plan. You gotta know what they want before you can start pushing. You want to treat someone who has never owned a cell a lot different than someone who wants to switch. "Oh, you're switching from AB&C Cellular? We have a lot of new customers who used to be with AB&C in the past. What got you thinking about switching?" Now you know what to emphasize in your sales pitch.
You want to steer them to the plans that match up with what they want, but let them think they are making the choice. Just like buying a car, if they tell you what they want to pay, you got 'em. Let's say you find out they want a $30/month plan. "We have THREE great plans for UNDER $30 a month. And one even includes a free GammaRay Z-100X phone! You can have that phone free with a plan that gives you 150 any time minutes a month. Or, if you need more minutes, our 250 minutes a month plan is only $26.99 a month and you can choose from any of these five new phones at a 40% discount. Would you prefer more minutes, or the free phone?"
No matter what they answer, they are just getting more and more comfortable in saying "yes". Notice that either choice is drawing them in. That's more effective than a question where they can just say no and walk away.
When you don't close a sale, think about what happened. What kept that customer from signing up? Some people are just wasting your time, of course, but you'll probably learn more from the sales you DON'T make than the ones you do. That way when you have another customer like that, you'll know what to do differently. If you feel someone is wasting your time and another customer comes up, give the first one a "Do you have any other questions?" They'll probably leave if they aren't serious, and you can immediately jump on the next person.