If people will walk past the outside ticket booth and come right in, then would the solution be to tear tickets just inside the door? Of course, you might have to check stubs again before seating. This would increase the manpower required, but it would maintain better control.
Anyone authorized to enter the lobby without a ticket, such as concession sale only or a parent picking up a child, would simply have to get permission from the usher at the door. And of course, anyone holding advance tickets would have no problems getting inside since they wouldn't have to worry about standing in line.
I think I would be pretty annoyed if you asked me to wait in line at the box office to show that I'd already purchased advanced tickets. Now if I had to pick up tickets purchased by phone, I might understand, but a "Will Call Window" is a great convenience. Even if the "will call window" is something as simple as picking up the tickets from the usher at the door, it might be something worth trying.
Just be sure to ask "will call" customers to show up a little early to pick up their tickets, something they probably wouldn't mind doing since they are the kind of people who do things in advance anyway. That way they wouldn't cause a delay at the entryway. If you make them wait in line again instead, they will be annoyed and cause the lines to be a little longer anyway just by their presence.
I can just imagine how special a customer will feel if they buy their tickets by phone and then show up, by-pass the ticket line, and meet the usher at the door who says, "Yes, Mr. & Mrs. Jones, you are right here on my list, and here are your tickets. Enjoy the show." This would be a great personal touch for those customers who make the special effort to make sure they have good seats by making the advance purchase and showing up early to pick them up. That would beat the AMC will call window anyday.