VoIP station calls can only be transferred using the ASAI or CTI interface to the Multivantage system. Transferring VoIP station calls with the VoiceXML <transfer> tag will not work.
The only way to transfer VoIP station calls within a VoiceXML application is to use the <object> tag. With the <object> tag, the VoiceXML application can direct control of the call to a TAS application. The controlling TAS application can then use the A_tran external function (for ASAI) or the ctiTransfer external function (for CTI) to transfer the call.
For more information on using the <object> tag, see Using the VoiceXML <object> element interface.