newcallmodel: link to transfer for sip call

+ Add the possibility to do a blind transfer
(transfer to a SIP number without calling him)
+ Add the possibility to do an attended transfer
(transfer to a current SIP call)

Change-Id: I88e09678e6edb0fea18172d4b84a6b3ae644284a
Gitlab: #362
Reviewed-by: Philippe Gorley <philippe.gorley@savoirfairelinux.com>