1. This is John Smith at the ABC bar.
Let us say someone is calling for a cab and he says that. Does that mean that he works there, or could it simply mean that he happens to be there?
Let us say I am at a bar and I am calling my cousin to tell him to come and pick me up.
Could I say:
2. I am Navi at the ABC bar.
3. I am Navi and I am at the ABC bar.
I would expect an employee to use "with."
I am not a teacher.
I would expect an employee to use 'from'.