What you are not seeing is that from the perspective of those on the street.... your dogs might just seem scary to t-h-e-m. You are speculating that it "might" be one person. Most places have caller ID which would identify the caller. Instead of looking negatively at your situation, why not look at it as an opportunity for something good?
Have you attempted to get ahold of the owners of the building yourself? I would not allow the landlord to be the go-between. I would deal with them directly. How long have you had your dogs in the apartment? Use calm logic. If you can prove that your dogs have been around for years without incident, perhaps you can persuade them to see your side.