I once called a plumber too, who instead of showing up with a snake, as I had requested, came with a bottle of Drano or whatever it is called here. He seemed to think the dumb foreigner would pay for him to poor that stuff down the drain. Some of these guys are re vivos!
I ended up pouring baking soda and 1/2 cup of vinegar down the drain, followed thirty minutes later with a pot of boiling water and plunging. That works.
Also the old buildings here have access to the drains in the floor. Once a month it is recommended to remove the plastic or metal cover, put on a rubber glove and pull out the hair and scum. Not pleasant but it works.
A final option, if your neighbors also have problems, is to call the city and get them to come by with the big truck that sucks out the garbage in the drains on the street. Apparently, in older neighborhoods the pipes are too narrow to handle all the runoff, hence the scum-sucking trucks.
Please try these options before using chemicals. The NaOH in these products can rust old lead pipes and eat through the cheap PVC pipes they sell here. This happened in my bathroom years ago and it caused a lot of water damage to the apartment below. You can also get chemical burns if it flies back up in your face as described above.
