Hello. If you don’t want to request a transition through the controlled airspace, you can fly above it and you do not have to contact ATC.
Alternatively, in the case of Class C or B airspace, you may fly under the outer shelf as depicted on your sectional chart.
In my case, if my route takes me through controlled airspace, I request a transition. Nine times out of ten it is granted, sometimes with an altitude restriction to keep me clear of ongoing flight operations.
For example, I often fly to the San Juan Islands in northern Washington. My route takes me through Whidbey Island Naval Air Station (Class C). I always request a transition because it keeps me over land in case I have to make an emergency landing.
Radio call would be as follows with the approach controller (or tower in the case of Class D airspace):
“Whidbey Approach, Cherokee 7428 Romeo, ten southeast, level 2,500, request transition, landing Orcas Island”
This provides the essential information, including aircraft type, distance, location, altitude, and destination.
Typical response would be:
“November 7428 Romeo, altimeter 30.20. (Or whatever it is), squawk 3627 (or whatever they give you). The controller may also use the words “transition approved“, but this is not necessary because once two-way communications is established between the controller and the pilot, you may enter the airspace (except for Class B which requires the controller to add words specifically authorizing you to enter the Class Bravo)
The controller may also assign you an altitude restriction based on flight operations within the airspace. For example, at Whidbey Island, Naval Air Station, there are often F 18 operations being conducted.
In this case, the controller may assign you an altitude restriction, or will deny your entry into the airspace depending on the situation.
After the controller sees your aircraft on his or her radar scope, they will come back to you with something like “November 7428 Romeo, radar contact 5 miles south of Penn Cove VOR, 2,500”
To this you would respond with, “position and altitude checks, Cherokee 7428 Romeo.”
Hope this helps!