Got a text message from 456 at 01:25 MST as follows:
You’re set! Line 16238001971 is now paired to your T-Mobile number <my number>. Visit t-mo.co/managedigits_pair to manage your DIGITS line.
I doubt if this is a new thing, but just in case, now we all know. I guess I’m supposed to be gullible enough to try to use that URL.