How to control another screen AND see/hear them!

Hello! I have used TeamViewer for several years, mostly to help my parents use their computer when I am not with them.  But now, I would like to see/hear them at the same time... and I tried this morning to do so. [Before I simply used my cell phone to call them, and then walked them through things as we talked on the cellphones.]

So this morning .. I was able to see/hear them, but they could not see or hear me. I am pretty sure my side was working (I could see my own video image, and see that the audio was working as well; but again they weren't seeing or hearing me.)  Is there any reference/directions step-by-step so that I can trouble-shoot this issue? Thanks in advance.

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