Ask HN: Anyone have a "sovereign" solution for phone calls?

I've been working on an SMTP/email server lately, and while Google and some others imply a lot of frowny-faces and put quotation marks around tests "passing" for not using a corporate relay, does at least let me communicate with the broader ecosystem.

Phone calls, however, seem like a tougher nut to crack. SIP URIs would let me kinda-sorta communicate with the broader ecosystem, but many phones and software seem to have dropped support for it; only a tiny % of those typically using the PSTN (that is, a "normal phone") would be able to call or receive calls to/from my addresses -- but it would be able to be directly and neatly integrated into the email server, which is a big plus.

I will probably still implement SIP URIs and VOIP support into the email server on principle, but I wonder if anyone has any alternatives to consider. Ideally, I would be able to communicate on the PSTN, but this seems like a lost cause.

Also curious about anyone using VOIP for work whether or not they allow or block SIP URIs from external networks. I maintained our VOIP server at work more than a decade ago, but it was a "side-project" due to working at an SMB where I wore many hats and couldn't specialize in anything; I wasn't even aware of SIP URIs at the time.