I think you want to pickup the call, not transfer. You will be able to transfer an active call on your phone... if you want to pick a ringing extension the button is pickup.
Now, fop2 beta improves the pickup code when it comes from a different context as the extension picking it up (like when it comes from a queue), so you might need to upgrade to the beta for call pickup to work in your environment.
You can download the beta from http;//www.fop2.com/downloads/fop2.tgz (for centos 5, 32 bits), If you have other distro let me know and I will point you to a proper download.
Best regards,
