It's AMEX and they are usually looking for money, even if they are in the wrong and you have made payment. Reasoning with them wont work, you can explain there's a mistake and they will promise to remove the automatic dialling from their system, they DON'T. They'll just keep calling, dozen times a day, every day.
They will also call you from multiple numbers, seem to have a practice of adding an extra number every day. Currently I am getting a dozen missed calls from 3 different numbers a day. Start at 7am and don't stop until after 9pm. Yesterday had total of 29 missed calls, ALL from Amex.