Conference call question

I was wondering this when I had a conference call today when this happened: person A calls person B. Person A then conferences in person C. They talk for a while and then person A hangs up. Persons B & C can still talk to each other, even though they were talking to each other “through A”. Is there still a circuit through A even though he hung up? Is a seperate circuit established from B to C when A confrenced in C? If so, who pays for that? To confuse matters even further, C conferenced in another party D (while A was still on the line). If it matters, A was in Seattle, WA., B (me) was in Washington DC, C and D were in Peoria, IL.