I'd love in SIP was more normalized for most people, so you just bought a data sim (and mostly used wifi) and didn't have to port a number around. I guess wireless market is so competitive the carriers really want to avoid this.
I ported my number out to VoIP.ms, and it included a four day waiting period... for no particular reason except to probably allow for some human to check a box.
The only reason you're not given access to it is because carriers are doing their best to protect their obsolete business model, and despite the appearances, Apple is fully complicit as well.
It's buried in the settings for the Phone app (the dialer) and AFAIK is often removed from OEM ROMs because of course it is.
As a VoIP engineer, it's a terrible soft client. It works, barely, and has basically no features beyond bare minimum calling. I've tried to use it repeatedly over the years but always ended up on commercial softphones like Bria or GS Wave.
But the call quality will never be as good as the native phone app as that gets QCI prioritization.
While it's true that SMS 2FA is flawed, that still isn't an excuse for letting customers' phone numbers being taken over by very unsophisticated attacks, sometimes even if notes are added to the account (or a PIN) that explicitly warn against such attacks.
Also, I'm not sure how much of "decades ago" is hyperbole but back in the day ISP/telco support was a great career path and would allow you to learn and move up the ranks towards a more technical position. Nowadays "support" in any customer-grade ISP/telco is a dead-end position that's there to be exploited as much as possible (in fact it's often outsourced to a boiler room abroad, probably right next to the tech-support scammers) and replaced by a new sucker as soon as you burn out. Obviously this kind of treatment doesn't attract the right talent nor inspire goodwill in said talent.