The "attached" session in SIPr is always associated with a fixed remote end point. Normal SIP provisions do
not change that. You create a attached session by using create_xx_session(ip, port).
You can have a detached session creat_xxx_session() where the remote target is evaluated by normal SIP mechanism and
can also be updated during signaling.
However there is no way to create a detached session while SIPr script is acting as a UAS. 
Need to have this enhancement.