I wish delta will implement peer to peer voip. if both user use delta then the voip will be available other wise it can show no available.
Actual behavior
I think it is possible because for p2p no need server. the chatting is enough as a messaging app but just the only thing messing is real time viop. hmm, jisti invite link is not enough.
So, I think jitsi is only solution. btw, In matrix (Element client) they add it very well for group call. If I just press the call button it automatically create the call without needing manually setup link also it is possible to join under this app no need extra app for mobile or no need to go browser for desktop.
Delta Chat works with more or less any email server as long as it supports standard IMAP and SMTP protocols, but email users normally don’t have a STUN/TURN server needed for call relaying.