This class represents a back-to-back (B2B) user agent (UA) that is used to attach an outgoing call (UAC) to an incoming (UAS) call. Normally the UAC call would be the client side of a call that is placed to an external UAS in this case it's the client side of a call to a UAS in the same process. The use for this class is to allow an outgoing call from a SIP Account to another SIP Account's incoming dial plan.