What, lively discussion.
Under 107, as others have said, you don't need to call when within five miles of an airport in Class G as you do under Part 101 (the hobbyist rules.) You do need a waiver, which can take up to 90 days but seem to be taking much less. The waiver will be for a specific geographic area within an airspace (e.g. I have two different waivers under one class D). Some of these waivers require you to call ATC, many do not. (None of mine do but as you have seen, some do.) Theoretically, these can be good for up to four years, but so far I've only heard of six months. Applying is pretty easy but if you get greedy they can and will be denied or take longer.
If you are using your UAS commercially then you need to register each drone, but all drones require a registration number (for hobbyists you can use the same number.) As others have said, it's five dollars and a few minutes per registration. It's not a big investment to just register them.
I'll also argue that operating under FAR 107 also provides you with a bit of insurance. The way that the rules are written, if you violate 101, then you are legally operating under FAR 107, even if you are violating those rules (it's confusing and annoying.) Taking the time to take the exam will help you learn more about the rules and what you can and shouldn't do. It's just a start but it will give you more information than flying under FAR 101. I'd argue it's worth the investment if you are flying anywhere near airspace or other aircraft.
You can find out more about applying for airspace waivers
here and
here. If it helps, you can find a discussion on the hobbyist rules
here. Sadly, it's far more confusing than it should be.
Good luck in your flying. Hopefully over time this will become less confusing.