• TransferringStateTimeout
  
  DWORD, time out in seconds for transferring line (blind call transfer) to become transferred, default: 20. If the call is not transferred within that timeout, the call returns to the transferring client, the line becomes "On Hold" again.
  
  Location: HKLM\Software\Swyx\Client Line Manager\CurrentVersion\Options


• BusyStateTimeout
  
  DWORD, time out in seconds for busy line to become inactive again if handset is onhook, default: 10
  
  Location: HKLM\Software\Swyx\Client Line Manager\CurrentVersion\Options
  (See explanation in this thread: Delay in clearing transferred calls)


• TerminatedStateTimeout 
  
  DWORD, time out in seconds for terminated line to become inactive again if handset is onhook, default: 4
  
  Location: HKLM\Software\Swyx\Client Line Manager\CurrentVersion\Options
  (See explanation in this thread: Delay in clearing transferred calls)

Reference: Swyx Partner Net KB Article Undocumented SwyxIt! Registry (kb2239) (Login required)