FreePBX & SIP device - how to do 3 way calling

I’ve configured queues. An agent receives a call. With the call in progress, the agent wants to outdial and conference the third party into the call so they hit flash, dials, waits for answer, and hits flash. Problem is that flash will only swap between the two calls.

The SIP device is an AudioCodes MP-124. 3 way calling is enabled, call waiting is disabled.