On the Signal app’s Advanced settings page, you can choose how voice and video calls are routed.
• Peer-to-peer WebRTC – The call travels directly between you and your contact, so no media passes through Signal’s servers, but each party sees the other’s IP address.
• Relayed through Signal’s servers – The call is tunneled via Signal’s relay server, which hides your IP address from your contact but exposes it to Signal.
In the relayed mode you can run the app over a VPN to hide your real IP from Signal as well. In peer-to-peer mode, using a VPN often makes it hard (and sometimes impossible) to establish a direct connection.