Exact same situation , you have to go to that exact bank branch in Thailand . Not any branch . The exact one it was opened . Then you fill in a form that they post to Bangkok to change the phone number .
The ask:thailand community, consisting of multiple Q/A groups with over 100,000 members, powers this platform. It is not an official government resource. Our members actively contribute to this resource, and while we strive for accuracy, we cannot guarantee its complete reliability. Assistance to travelers is provided as a community service.