The whole situation is a huge mess but a very interesting one from a nerds perspective. On the one hand we're dealing with NAT - a bandaid fix for a lack of IPv4 address space that was supposed to be a stop gap until everyone was using IPv6. Well here we are in 2024, 34 years since address exhaustion was predicted, and 5 years since RIPE allocated it's last /22, and IPv6 is still far from a de facto standard. That's 34 years of exhaustive planning and implementation from netops for a shift in technologies that still hasn't taken hold. NAT breaks the end-to-end principle of the internet and everything we see in this thread stems from that fact. It doesn't help that NAT has essentially become a safety net, preventing people from accidentally exposing their devices to the wider internet.
On the other we're dealing with SIP, a protocol that was formalised in RFC2543 25 years ago, being scaled up to serve an entire nation. The fact that we are assigned a fixed proxy server hints at some really ugly compromises on the back end to ensure adequate load balancing and reliable functionality.
That we have to care about anything more than a server URL, username and password to use our own equipment is an abject failure in terms of end users. Especially considering that planning for 21CN started in 2004(!). Including a widget to transparently connect a PTSN phone to CPE is the icing on a very bitter tasting cake, robbing the majority of customers of the ability to choose their own hardware. The service providers network ends at the modem - everything beyond that should be entirely at the end users discretion.
In the intervening time, Apple, Skype and others have designed and iterated software that provides a network agnostic means to place audio and video calls on a *global* scale across billions of devices. Facetime even provides a layer to seamlessly connect people using their legacy phone numbers and it doesn't care if you're behind NAT.
Honestly it's like they just couldn't be bothered to do better. The current implementation is so half baked it's more like a token gesture, they know the idea of having a land line is dying and this is just a stop gap until it goes away entirely. They didn't even mandate feature parity with the old PTSN network. No voice mail service, really?
