About handling of VoIP and ordinal telephony calls. The right way is to use Telepathy framework.
So the first thing that you need is to implement Telepathy Observer interface ( http://people.collabora.co.uk/~danni...r.clients.html http://telepathy.freedesktop.org/spe....Observer.html ).
To handle VoIP calls only you need to create filter for TP_IFACE_CHANNEL ".ChannelType" = TP_IFACE_CHANNEL_TYPE_STREAMED_MEDIA and TP_IFACE_CHANNEL ".TargetHandleType" = TP_HANDLE_TYPE_CONTACT.
The observe method will be called when new challes creates.
Incoming call can be identified by TP_IFACE_CHANNEL ".Requested" property of channel.
You should set hook to "members_changed" signal of channel to handle switching call to active state.
The caller identifier sored in TP_IFACE_CHANNEL ".TargetID" property of channel.