Nextcloud Talk - installed but no participants?

Dear Forum,

I just activated talk on my Nextcloud 16 and I would like to use it for video calls between Canada and Germany.
When I click the talk icon, I can start a “new conversation” and invite participants.
But all participants don’t have the talk icon (looks like a Q).
Why is this so?
I can copy a link and send this to somebody else. In one case this person could do an video call with me - great.
In another case, there was only the posibility to chat - hmmm…
Is this because of the different browsers?
Does everybody who wants to use talk video calls the mobile app?
Is there a wiki or manual for talk?
I think the instructions for this app are very rare… I could really need some more explanation what to do.

Thanks for your help