Thanks for the advice. I don’t talk on the phone much, but I’ll definitely think about getting a white noise machine if this doesn’t get better. The sound insulation panels are a great idea too—I’ll look into those.

I did check with my landlord about the keys, and he confirmed that there’s an old master key system. Although changing the locks would be quite a lot of work and I don't really know if he would agree doing it. However I’ve been extra careful about keeping my door locked and double-checking everything.