Before operating a small unmanned aircraft in Class B, Class C, or Class D airspace, or within the lateral boundaries of the surface area of Class E airspace designated for an airport, 14 CFR § 107.41 requires the remote pilot in command to —

Why: 14 CFR § 107.41 states that no person may operate a small unmanned aircraft in Class B, C, or D airspace, or within the lateral boundaries of the surface area of Class E airspace designated for an airport, unless that person has prior authorization from ATC.

• Filing an FSS flight plan is a manned-aviation IFR/VFR concept; § 107.41 requires ATC authorization, not a filed flight plan.
• Part 107 has no requirement to call an airport's FBO; the authorization § 107.41 requires runs through ATC, not airport ground personnel.