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A call B
B is on the phone with C

so on A side:
sorry, that person is already on phone, please press 1 to be warned when he hangups.
if A preses nothing or hangup, it does nothing.
If A presses 1, he hangups next.
When B hangups with C, * call A.
when A answer, it tells, C has left the phone, im calling him now.
* calls C 
when C answer, A and C are bridged.