Attended transfers to an external c-leg is breaking 3-way conference functionality for me. Like this:

<action application="att_xfer" data="sofia/gateway/onesource/${attxfer_callthis}"/>

The c-leg is established and hanging up the originator's leg will successfully bridge the other 2. However, pressing 0 on the originator, which should establish a 3-way conference, actually hangs up the c-leg. Is there another method by which I should specify the outbound leg for att_xfer?