The telephone is an old technology, and it can cause many problems in the modern workplace. The fact that it is a public alarm (ringer) is quite disturbing to those around you. Additionally, it’s not easily managed to help during times of focused concentration and productivity. In most places that I have worked for the past 10 years, people would only use the phone during an emergency or if you request the call via email or chat first. In other words, I believe your boss is using proper office etiquette that takes into consideration those around you, provides you with the respect and trust that you know how to manage your time, and that you may be in the middle of something very important. This is completely normal, and I would view this as a very considerate and respectful thing that your boss does.