You wouldn't need to blind an experiment like this, I'd think. Placebo won't prevent cancer, and nocebo won't cause it. (Besides, I don't see how you can blind it. Not like someone's not going to know whether their phone works or not!)

You could try recruiting your control cohort from the National Radio Quiet Zone. I believe it's quite thinly populated, though.

Radio energy drops off pretty quickly. If the question you hoped to answer was does RF at any level higher than the background from the Sun + cosmic background cause cancer then that would be difficult, but if the question was does holding a smartphone cause cancer it would be relatively easy. It would also be easy to give people a device that either transmitted rf or didn't with no outside indication. Also really easy to do this in mice or the like. So far all indicator are that for the sort of frequencies we use the only impacts are heating

You put the control group in the United States National Radio Quiet Zone.

At this point you can't even talk about "blind", not to mention "double blind".

You'd also have to put the experimental group there, too.

You can't. The Zone exists to enable sensitive radio astronomy; if you radiate without authorization, they DF you and come tell you to stop.