I have a telephone system with all V.O.I.P. lines.  I connect to the telephone network via a Yealink W56P/W52P system. There are no POTS lines involved. Will the "hangup" feature work with this? My understanding is that a modem can only "hangup" a physical line. What is required (assuming it is possible) to enable the sip2ncid gateway to handle the hangup function. 
Thanks
Len W.
            
  
  
    
  
            
               
        
            Anonymous
        
    
            
          
As you noted, the current hangup feature uses a USB voice modem to pick up, optionally make an announcement and then hang up.
It might be possible for sip2ncid to be more than an observer but we need someone with more SIP expertise .