It seems that asterisk 18.104.22.168 doest not like REFER so Siptapi does not work. Do you know of 1.6.0.x version working?
Siptapi works well with 1.4
Probably the configuration has changed, I am quite sure it should work with 1.6 too. I always use promiscredir=no (sip.conf) and set the explicitly the TRANSFER_CONTEXT in extensions.conf, to handle the transfer in a dedicated context.