You should call people by their correct or preferred name.
Debbie's coworker has recently come out as trans and has asked to be called by a name that matches their true gender.
Debbie wants to call her coworker.
Debbie calls her coworker and uses their correct new name.
Debbie calls her coworker but uses their dead name instead of their new correct name.
Debbie's coworker feels respected and validated by the use of their correct name.
Debbie's coworker feels disrespected and invalidated by the use of their dead name.