That was a spanking well deserved for your son. And that attitude would have also made the spanking a lot worse in this home, as well. That reply of "Because I am 18" in response to not calling, would have made me a lot more upset, as well. He needed a reminder of the rules and who is still in charge.
I have had to give my share of spankings for curfew violation. Some with my son and with Andy, but it happened more often with my girls. I had a very similar rule as Cherie in they had a 10 minute grace period to make it home. Unless they called and had a good reason why they were going to be late. And just letting me know "Mom, I'm running late" was not a good reason.
Your oldest definitely needed that good (sore) reminder while sitting in church. Good for you, Teresa.