I’m not looking for jobs because of my age 78f but I’m 100% fluent on the phone and even in person as long as the topic is stuttering! It’s as puzzling as your situation because we know it’s possible to speak fluently, except when in the situation of an interview. It’s partially physiological and part psychological. I think there’s the dopaminergic circuit damage which predisposes us to stutter as well as whatever psychological damage we experienced. There has been improvement through speech therapy and psychological therapy but what we’re all awaiting is medication to modify the D2 neurological circuit. That would give everything else a chance.