Absolutely, I remember owning a nokia n85 12 years ago which had native SIP support. I wish ios and android had this.
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.