title: SIP Register and SUBSCRIBE
body: <p>I wanted to understand nuances around REGISTER and SUBSCRIBE. In most you would expect UAC to register and then subscribe. Can UAC be Subscribed but not registered..? or can UAC be registered with more accounts than it is subscribed to. </p>
  
  <p>E.g. REGISTER with 5 accounts and be SUBSCRIBE on 3</p>