Here is a proposal for expanding VoiceCallManager's DBus interface with 
call related Supplementary Services signals.
The implementation would be based on the functionality provided by the SSN
atom (handling the CSSU codes). 

---
 doc/voicecallmanager-api.txt |   54 +++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 53 insertions(+), 1 deletions(-)

diff --git a/doc/voicecallmanager-api.txt b/doc/voicecallmanager-api.txt
index 2bf9ded..5212eac 100644
--- a/doc/voicecallmanager-api.txt
+++ b/doc/voicecallmanager-api.txt
@@ -123,7 +123,59 @@ Methods		dict GetProperties()
 			'*', '#', 'A', 'B', 'C', 'D'.  The last four are
 			typically not used in normal circumstances.
 
-Signals		CallAdded(object path, dict properties)
+Signals		CallForwarded(object path)
+
+			Signal sent during call setup when the incoming call is
+			a forwarded call. The signal contains the object path of
+			the call in cause.
+
+		ClosedUserGroupCall(object path, int32 index)
+
+			Signal sent during call setup when the incoming call is
+			a Closed User Group call. Since the user may belong to
+			multiple groups, the index indicates the group that is
+			applied to the incoming call.
+
+		CallOnHold(object path)
+
+			Signal sent during voice call when a call has been put
+			on hold by the remote party.
+
+		HoldRetrieved(object path)
+
+			Signal sent during voice call when the call has been
+			retrived from held state by the remote party.
+
+		MulipartyCall(object path)
+
+			Signal sent during voice call when a call has been
+			added to a multiparty/conference call.
+
+		HoldReleased(object path)
+
+			Signal sent during voice call when a call put earlier on
+			hold has been released.
+
+		CallForwardCheck()
+
+			Signal sent when a forward check message has been received.
+
+		CallInTransfer(object path)
+
+			Signal sent during voice call with the remote party in
+			alerting state in explicit call transfer operation.
+
+		CallTransferred(object path)
+
+			Signal sent when the call has been connected with the
+			remote party in explicit call transfer operation.
+
+		CallDeflected(object path)
+
+			Signal sent during call setup when the incoming call is
+			a deflected call.
+
+		CallAdded(object path, dict properties)
 
 			Signal that is sent when a new call is added.  It
 			contains the object path of the new voice call and
-- 
1.7.0.4