[OpenSIPStack] How to send CANCEL msg? And why CANCEL is absent in the list ALLOW of INVITE msg ?

[H.Kropf <mai...@gl...>]

I wish to cancel an outgoing call which the remote side does not answer 
longtime (ringin on remote side). For this purpose I should send message 
CANCEL.
For this it is necessary to call method CallSession:: SendCancel()?
But how to obtain CallSession object?

Some softphones (for example EyeBeam set) announce method CANCEL in the 
list "Allow" of message INVITE. Why OSS does not do it?