There is a new targeted Amex Offer to save $100 off $150 at AT&T Wireless through 10/31/17. One-time payments to your personal wireless account should work.
To check if you were targeted, login to your American Express account and check your Amex Offers at the bottom.
You can add this deal to multiple credit cards if you have the offer available on more than one account. Make sure you open each card in a separate tab before adding it to any of your cards.