summon, recruit, musical note
徵 centers on calling someone to come or to serve: summoning, recruiting, or drafting. The musical note sense is a distinct, specialized meaning that does not clearly connect to the summoning idea.
徵 is the traditional form of 徴. It combines 彳 (movement) with a phonetic component that likely contributed the sound and the idea of calling or summoning. The musical note meaning may have developed from a separate usage.
The left side 彳 suggests movement or action. Picture a person walking to summon others, like a recruiter calling people to gather. The complex right side can be seen as a banner or signal used to call people.
For チョウ, imagine a town crier named Chou walking through the streets to summon recruits: Chou -> チョウ, and his call brings people together.