Thank you zonoma. As ex ATC (for 45 years) I consider my doubts are just as valid. I note that you exclude the possibility of the aircraft taking the wrong call for a frequency change. That also happens quite frequently. As for your suggestion that
It is possible for the aircraft to have dialled in a wrong frequency, checked in as normal, and for the ATC unit to acknowledge without realising that the aircraft should not be on their frequency.
this would indicate that you have no knowledge of what functions an ATCO has to perform upon receiving an initial call on his sector. We are not talking procedural control over certain parts of Africa here!