When flying IFR, a pilot "reads back" that is repeats back all the instructions we receive from air traffic control, to demonstrate that we have heard and accept the instruction, and to give the controller a chance to notice if we have heard incorrectly, or if the controller misspoke. It gets a little silly sometimes: the pilot asks for a particular clearance, the controller clears them for it, and the pilot reads it back again, but it prevents us from descending to five thousand when it's only safe down to nine thousand. When I say pilots talk to "air traffic control," we are only sometimes speaking to someone who can look out the window of a tower and see us. We're usually speaking to someone who can see an electronic blip on a computer screen, generated in response to radar return and transponder code information from our aircraft. We may even be talking to someone who merely has a copy of our flight plan and has keep track of aircraft based on our position reports, in an area with no radar coverage. They probably have a map of their area and some cool computer tools to help them. The sky is all parcelled out vertically and horizontally into different classes of airspace, some requiring air traffic control and some not. A pilot might first talk to the clearance delivery position, then call ground for taxi clearance. The ground controller instructs the pilot to contact tower at the hold short line. The tower controller passes us on to departure or terminal and then terminal instructs us to switch to Centre. All the way across Alberta, and much of northern Canada we're talking to Edmonton Center, but as I cross Alberta I'm progressively switched between frequencies, so I'm always in range of the antenna that transmits and receives on that frequency.
The controller might say, "ABC, Contact Edmonton Centre on 132.75" and then I say "ABC, 132.75." I tune 132.75 and then call that controller, saying, "ABC, one five thousand." If a pilot neglects to acknowledge a frequency change, and just goes straight to the next frequency, the poor controller that directed her to change has no way of knowing whether she changed or just fell asleep. And of course she has to check in on the new frequency or the new controller doesn't know that the change has been made successfully. Sometimes the controller's wording is "Contact me now on 133.4." That means the one controller is managing multiple frequencies. "Contact me now" means "Switch to my other frequency" Sometimes they just say that, too, or "Switch to my frequency ..." When things aren't too busy, in remote areas or overnight there are fewer controllers, so one person may manage all the frequencies in a vast swath of airspace. Sometimes the controller sets it up so that all the pilots on the various frequencies she is controlling can hear the controller regardless of which frequency she is broadcasting on, and sometimes we can hear the other aircraft on those frequencies too. This makes it easier for us not to call on one frequency when the controller is speaking or listening on another. Sometimes the frequencies aren't paired this way so we do sometimes talk at the some time as the controller is busy on another frequency, and she has to ask us to go again.
I learned to fly IFR during the day in busy airspace where all the frequencies had their own controllers. I would read back my frequency change instructions, change frequencies, and check in with the next controller. And then I went north and honed my skills in remote areas in uncontrolled airspace where there were no Centre controllers to talk to, just pilots talking to one another, reporting when we changed altitude or passed over waypoints. So it took me a really long time to notice something. Here comes the confession.
If a controller says, "Contact me now on 132.75" you don't need to read that back on the original frequency and then switch frequencies and check in." You can just dial in 132.75 quickly and say, "ABC on 132.75." This did not dawn on my for the longest time. I noticed a pilot doing it one day, when the controller had paired the frequencies, so I could hear the pilot given the instruction to switch and simply acknowledge it on the new frequency. I think most people do this most of the time. Do you?