Your current group of teenagers are very close in age and are mostly friends with each other. A day came when two boys, one old enough to drive, the other a year younger, were in trouble for the same incident. Instead of grounding one, you paddled them both. The next time trouble occurred with an older boy, complaints of favoritism were lodged, and you adapted a new system, where the boys usually have a choice of restriction or corporal punishment. The system has worked well, so you've continued with its use.
Barry and Josh returned from debate camp today. The rest of your family had seen Guardians of the Galaxy last night and loved the movie. Because they were talking about it a lot, Barry and Josh announced they were going to see it that day, instead of waiting several days to see it in Imax with the rest of the family (you understand that, since you hate spoilers with a fiery passion you normally reserve for people going under the speed limit in the left lane).
After unpacking, Barry and Josh tell you they're going out to visit some friends, and will be home for dinner. They arrive on time, wash up, and join you in grilling burgers. At first, they're telling you more about what happened at debate camp and discussing some issues with you, but then they switch to talking about how great Guardians was, and comparing favorite parts.
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"Yeah, we just got out."
"You said you were going this evening."
"Yeah, but we figured everyone would be around tonight, and we wanted to do some gaming and stuff."
"Why didn't you tell me?"
"We were home on time!"
"Do you have cell phones?"
Of course you know they do, since you're paying for them. They don't try to deny it.
"Isn't the main purpose of having a cell phone so I can contact you if I need to?"
"One of them, I guess... I mean, not just you specifically."
"And how can I do that if your phone is turned off for two and a half hours, and I don't know why? What would I think if I tried to call you, couldn't reach you, and had no idea why?"
While they were home on time, didn't violate any boundaries, and there isn't a rule about having to notify you before turning off the phone, you all three agree that it's just common sense (and common courtesy) that they should have let you know they were changing their plans and would be out of contact for a while.
Normally, you give the boys a choice between three swats on their boxers or two bare for 'regular' offenses that require a consequence. A lighter consequence would be two on boxers or even on pants. You could also use some type of restriction, but since you're leaving for the amusement park tomorrow, that wouldn't be too easy.
What should you say next?