I think that there is a bug involving "barge-in" see:
http://www.obitalk.com/forum/index.php?topic=5031.msg32468#msg32468However, I'm not sure that is the problem here. This box needs to be checked:
Physical Interfaces -> PHONE Port -> Calling Features > EnableLINEPortBargeIn
To transfer a call to your softphone, you need an address for the softphone. Assuming it's on the same subnet its address might be softPC@192.168.1.13:5476 This can be put in a speed dial slot in your OBi as sp2(softPC@192.168.1.13:5476), say speed dial slot 6. I'm assuming that you will use sp2 to dial the speed dial. sp2 needs to be set up for SIP. To transfer, put the original call on hold, dial 6#, answer call on softphone, clear down OBi phone.