Wording of "What's a good phone number for sending a reminder over text message?"

[jwbowers]

How about "What's a good phone number we can use to send you a reminder as a text message?"
or

"Would you like a text message reminder? If so, let us know here what number to use."

(By the way, I haven't seen any studies on getting people to realize they need a reminder. We could randomize this prompt a bit to get some leverage on that more general question. I don't see Nat on github otherwise I'd @ mention him.)