The only app you can use to transform your cellphone into a webcam is Tellybean , but the app is outdated, the security certificated is down on their website and it seems to be an abandoned project, you can use a webcam with TrueConf, however, let me tell you the current problem with Android TV and Android AOSP with webcams:

First of all you must take into account that UVC (Universal Video Class) webcams are the ONLY ones that work with Android TV, you can check a list here , recommended webcams are the Logitech C270 which I own and it works correctly. You can also use Jami ( jami.net ) to make videocalls.

Second of all, oficial webcam support was provided starting with Android 9 Pie, however, the Nvidia Shield and the original Xiaomi Mi Box were the only devices (as far as I know) that supported UVC webcams when they were released.

When I bought the original Mi Box back in 2017, I was able to use my Logitech C270 webcam with Tellybean, because the app supported the remote control, it was independent from smartphones and the microphone worked as excpected.

When Android 8 arrived, webcams were still supported but for some reason the driver built into the firmware was failing to get the microphone working, I thought this problem was present ONLY on the Mi Box but then I realized it was also present on Android 9 because I bought another Android TV device from another brand (Mecool KM9 Pro) which came with Android 9 and I was surprised that I found the same issue with the microphone I got on the Mi Box with Android 8.

I later bought a Mi Box S with Android 8.1 and the problem was still there and now that the Mi Box S has Android 9 I was able to confirm the problem is still present, which means every single Android TV device out there with android 9 is plaguing with the same problem except... NVIDIA SHIELD DEVICES!!.

Now, I already created a bug report at Issue Tracker website about this ssue, but I need more people to chime in, adding commentaries about this problem to finally call Google's attention to fix this problem for good!!!. Could you please add your comment about your experience with this problem? please!!! here is the link:

https://issuetracker.google.com/issues/145900984

Thank you in advance.

Very actual question for me as well!I have recently posted this: https://www.reddit.com/r/AndroidTV/comments/g8yf4s/skype_calls_via_android_tv_box/
but haven't got any substantial feedback, except the first comment, check it!

Make calls from the phone and then screen cast to MiBox?

When connected to Mi Box, your Android phone will be detected either as a USB drive or a modem, but not as a webcam. Most likely, it won’t work the way you expect. I’d suggest using a standard webcam, something like Logitech C920 which is supported by TrueConf.