When flushing the toilet does water come up in the TUB?

When you fill the sink to the top and let it down does it flow into the tub?

These are two signs of a partial blockage BUT if neither of these are the case you may have a blocked Vent pipe.

E plumber ASSUMED this was a partical blockage without bothering to ask if this bathroom is located at the same level as the main sewer.

He could have asked if this was back to back of other fixtures or is this on the main floor and you have a basement.

Sight unseen another plumber would have to ask the right questions I would imagine, BUT E plumber he is right on so why bother to go into asking questions