The good news: putting my phone number and a close friend's phone number on Cassie's ID tags worked!
The bad news: I gotta trek back to the dog park to pick up Cassie's tags.
Because of the way they lay on her collar, they don't jingle when she walks, only when she shakes or (as happens from time to time) rolls over to let me demand that I rub her belly. So while walking back from the dog park, I had no idea that she didn't have her tags anymore.
Sigh.