Transfer and conferencing are not supported in Version 1.3.0 of VOCAL, so it is best to leave these parameters alone:
Ua_Xfer_Mode string Off Conference_Server string firstname.lastname@example.org
Turns on transfer or ad hoc conferencing. The options are off, transfer, or conference.
Specifies the URI for the Conference server. The URI consists of the conference bridge number and the IP address of the Proxy server or the conference bridge itself (if no proxy is being used).