Hello,
this is what I found in the "IP Office Public SIP Trunks Overview and Specification" PDF. The document is from 2013, I will see if I find a more recent one.
"For public SIP trunks, IP Office supports only consultative call transfer using REFER.
Consultative transfer is also known as Attended. With consultative transfer, the TransferOR
puts the Primary call on hold and establishes a Consult call to another party. After the
consultation, the TransferOR completes the transfer, causing the TransferEE to connect to the Transfer Target, thereby replacing the Transfer Target’s call with the TransferOR."
So it seems that IP Office does not support blind transfer over SIP Trunk.
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