How to place BackToBack call on hold?

I'm new to Lync UCMA. We're working in UCMA 3. I understand (more/less) that you have to adjust the audio routes manually when a BackToBack call is in a conference. However, I'd like to know how to place a BackToBack call on hold that is NOT in a conference. The Call1.Flow and Call2.Flow is null in a BackToBack call (which I think is probably correct since the RTP is endpoint to endpoint). So how do I issue a BeginHold? I'm so stuck on this...

Thanks!