Part 107 pilot failed to receive authorization to conduct a flight within controlled airspace. Flight was completed safely.

Narrative

I was on-site to perform a photo mission to take some photos of [the roof top] of [an] elementary school. We are doing an [equipment] replacement project for this school. The work will involve some work on the [rooftop]. I called the ZZZ Tower/ATC to let them know about the flight prior to the flight. After speaking to them; I assumed that I was authorized to make the flight from having informed them about the flight and based on the fact that they did not tell me that I was not authorized to make the flight during the call. I performed the drone flight to an altitude of no more than 150 feet AGL for a duration of no more than 18 minutes. That is the time from the first call to the second call to the Tower/ATC to let them know that I was done with the flight. While I was operating the drone; [the] Manager of Drone Operations called and left me a message. I did not know about his call until after I made the flight. After the flight; I returned his call. He identified himself as the person in charge of the Drone Operations for ZZZ Tower. He indicated that I was not authorized to make the drone flight. At that point; as stated above; the flight was already complete; and I made the flight assuming I was already authorized to make the flight from my earlier conversation with the Control Tower Operators. I explained this to Manager of Drone Operations. I also asked him what I should do to ensure that I was authorized to make future flights in the area of ZZZ. He indicated that I would need to contact him for future authorizations. I indicated that I would do that for any future flights in the area of ZZZ. The call was ended.