Important thing to add - in Extension Phone Provisioning Settings please add Open VPN Enable and Path to Open VPN tarball for phone. I have succefully connected Yealink T28 and T26 to 3CX Server via OpenVPN! But after phone autoprovisioning, the phone loose its OpenVPN settings. Also, distributing of tarballs to each phone is hard job. What you think? P.S. I am working on Yealink - 3CX OpenVPN manual.