Sets a flag that indicates that this Dialog is part of a BackToBackUserAgent. If this flag
is set, INVITEs are not allowed to interleave and timed out ACK transmission results in a
BYE being sent to the other side. Setting this flag instructs the stack to automatically
handle dialog errors. Once this flag is set for a dialog, it cannot be changed.
This flag can be set on a stack-wide basis, on a per-provider basis or on a per Dialog basis.
This flag must only be set at the time of Dialog creation. If the flag is set after the first
request or response is seen by the Dialog, the behavior of this flag is undefined.