Overall flow looks like:
```
telegram->ntgcalls(opus dec under the hood)->opus enc->pion->xmpp
xmpp->pion->opus dec->ntgcalls(opus enc under the hood)->telegram
```

In code:
```
Call
├── Bridge (call state)
│   ├── Callee: tgsig|xmppsig.Callee (signaling)
│   └── Caller: tgsig|xmppsig.Caller (signaling)
├── TgToXmpp (audio)
└── XmppToTg (audio)
```