Data point:

- Started off as a long time AT&T customer.
- Applied for and received Cit AT&T Access More credit card.
- Went into an AT&T branded reseller store in nearby state where I was on business to purchase a 6S Plus.
- Went into an AT&T company store in state where I live to activate phone on my existing AT&T account.
- After approx. two months of continuing service, ported my AT&T number to a competitor due to the value offered by the competitor's promotional rate.
- Only then got around to mail copy of my phone purchase invoice annotated with my credit card number, full name, and AT&T account number (in case they could no longer look it up using my already-ported phone number).
- Received $650 credit transaction to Access More credit card within 2 weeks.

Arguably I made multiple mistakes in execution, but all ended well.