Perhaps I could suggest a solution, but I do not understand the algorithm you need to get.
I understand correctly what you want to do?
The database has a list of phone numbers (suppose of 3 pieces). On the administrators window there is a Call button and LabelN (N - nomber of administrators). When the first administrator logs in, he presses the Call button and a call to number1 occurs, which is displayed in Label1. After completion call, the first administrator presses the Call button and a call is made to the number2, which is displayed in Label1. At this time, the second administrator enters the system hi see Label1 = number2 and Label2=“no call” with are displayed on its screen. The second administrator presses the Call button and Label2= number3.
That is, the first and second administrator on his screen sees Label1 = nomber2, Label2 = nomber3. The first administrator ends the call and leaves for lunch Label1 = “no call”, the second administrator ends the call and presses the Call button. After that, he sees on the screen Label1 = “no call” and Label2 = number1