> VoLTE and VoWIFI, for example, are both wrappers over SIP if I'm not mistaken

Not wrappers. VoLTE is literally SIP with extensions. And VoWiFi is literally VoLTE over IPSec/IKE with EAP auth, so SIP over IPSec. Really just bog standard stuffs.

> – pretty complex, but still using modern tech

Not sure about "modern", but yeah pretty much every actor uses SIP one way or another. It does have a few old stuff (one SIP extension of VoLTE is to allow SIP-negotiated IPSec tunnel. For HMAC it supports md5-96 and sha1-96 and that's all.), but nothing that makes you scream of over-complication.