You'd trust the people at an average fast food joint (predominantly minimum-wage teenagers) to keep track of your phone? You have much more faith than me...
I'm not sure where you're from, but in the united states, employers can pay an hourly rate of $2-ish to people who are expected to collect tips. The stereotypical fast food kid actually has a better hourly rate.
Since when is the definition of a restaurant worker someone who does not make much money? Especially those not working fast food?
Naturally this only applies to the waiting staff, but then again, the chefs aren't going to be the ones handling your phone.