Between library email treated as spam, SMS messages blocked by the carrier, phone calls not received, typos in patron contact details or a misconfigured Notice Preference field, there are multiple potential failure points when notifying patrons that their holds are ready for pickup.
So don’t make things worse by misconfiguring a loan rule like I did:
I failed to specify a message number from the Text of Circulation Notices table. Same goes for the Text Cancel Pickup field below it. The loan rule is used by a class of very popular laptop+hotspot kits, and thus you can imagine how many patrons have been poorly served by this misconfiguration.
Credit and gratitude to Tim Kong at Innovative Support for spotting this omission! As you can imagine, this prompted me to check the 100+ other loan rules which are Holdable = Yes.