F-Droid

meaning both sides could talk at the same time

You're describing "full-duplex" communication (or sometimes just called "duplex").

In practice, that's not always a "continuous line of communication". As long as messages can go in both directions at the same time, it's full-duplex.

Compare this to a walkie-talkie. With walkie-talkie, you're constantly transmitting or receiving. But when you push the button to speak, you can't hear anything until you let go of the button. This is because it can only transmit or receive (half-duplex).

As a general rule, VoIP should be duplex. Voice notes (on any platform) would be half-duplex (you can't listen and record at the same time).

Something that can muddy the waters with VoIP and other real-time voice methods is noise cancelling and bandwidth/data reduction methods. Your device might try to reduce wind noise, avoid sending data when you aren't talking, etc. In doing that, sometimes it interferes in a way that makes your brain feel like your call isn't a "continuous line". That might sound like a completely dead line (no line static) that makes you feel like the call cutout. Or the other person's noise reduction algorithm might cut or clip some of their audio.

Both sides can talk at the same time during VOIP calls. While Simplex can send audio or other files you can also tap the phone icon in your contact. This will make a regular phone call or video call with two way communication. For me, calls have worked the best out of other open source apps other than Signal. Only problem I have had was echo when using speaker phone. Regular calls work fine.

If you were asking about the network layer how things work. There are small packets of information going in both directions. Dropped packets will cause short lost audio. But normally all the small packets are put together into a continuous two way communication.