There is still a STUN/TURN server behind the scenes. For Matrix it’s relatively easy to deploy, because most users are on the matrix.org server and there is a clear howto on how to add TURN to your synapse server installation:
https://github.com/matrix-org/synapse/blob/develop/docs/turn-howto.md
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.