Typically when clearing an IFR airplane to take off, a tower controller asks the pilot to contact the next air traffic controller when reaching a particular altitude, or just "airborne" (right after take off). Sometimes that instruction is given after takeoff, but it's easier and safer to talk to us before we have advanced the throttles to take-off power. If the departure airport is uncontrolled, we'll check in with the first available IFR controller, as soon as we are clear of aerodrome traffic we may need to talk to, and reception altitude permits.
We have an expression "aviate, navigate, communicate" that reminds us that flying the airplane and being where we're supposed to be is more important than talking on the radio, so that altitude may come and go, and we may be aviating or navigating instead of making that call. The system allows for a fair amount of ambiguity here, as we may take off, change to the next frequency, and find it congested with other traffic checking in and receiving instructions. We have an assigned departure route, and in fact a whole route to destination to fly without ATC assistance, but we are expected to call promptly when able. The other night we heard a controller looking for an aircraft that had taken off but not checked in with him yet.
"Northflight 27 on frequency?"
The "on frequency?" question is a pretty common call. You might switch to a busy frequency and not have had a chance to get a word in edgewise before the controller wants to alert you to traffic or give you a descent, so the controller elbows a path through the crowd of calls for your response. On this occasion, though, the frequency was quiet. It was night, and an efficient controller was just trying to get every blip on his screen going in the proper direction.
There was no response to that call, so the controller tried again a short time later.
"Northflight 27, Sumspot Departure."
Still no response. "Sumspot" isn't the name of the agency any more than "Northflight" was the actual carrier, but you get the idea. The call that finally made us laugh was:
"Northflight 27, are you paying attention yet?"
Northflight wasn't. I suppose a controller has to watch how cute he is with these wake-up calls, because if Northflight 27 had departed from Fleabush runway 17 and then turned left instead of right and come to an untimely end smeared against a hill, the controller's cute remarks would be part of the accident investigation. I find it chilling to read the transcript of the Egypt Air 990 communications as ATC does their job, trying to re-establish communications with an aircraft they don't yet know has been destroyed.
In this case, the pilot of Northflight 27 innocently checked in a minute later and was given instructions with no hint of the amusement the controller had provided for the frequency, at his expense.