Specifically, Signal's secured VoIP is Ogg Opus (better performance than the technology your phone uses to make actual phone calls) but in CBR mode (compression but with a fixed ratio so that a frame of audio is always the same number of bytes, thus a MITM attacker can't determine anything except how long your call lasted)

An unencrypted voice chat app could do VBR Opus, which would need slightly less bandwidth on average for the same quality but might mess with flow performance estimators in the network and make the experience worse by mistake.

One of the inadvertent benefits of Signal's strong centralisation of control is that they could just go "Opus CBR is great, we're using that now" and do it. No multi-year phase-in with most calls still being "legacy", no need to accumulate "buy in" from third parties who might not prioritize this work, they just did it.