with all the VoLTE / VoWifi / WifiCall talk, I was missing a SIP howto. If you have great connectivity but no voice service into or from the classical phone network it’s good to have an alternative.
The built in client is bare bones, audio codecs are what you’re used to from 3g networks. But it works! is lightweight! has your contacts already!
To get an SIP account, use voip.ms or callcentric in the US and enter details. If you want to rely on the service, please read their E911 page how emergency service is routed (to the address of your registration mostly) and how text messages work(*).
(*) voip.ms has text messaging since mid 2020. I haven’t tested if the /e/ messages app (qksms) can use the sip account to text. If not, there is a separate app: VoIP.ms SMS | F-Droid - Free and Open Source Android App Repository
The components the AOSP Dialer SIP implementation is built upon is on its way out by Android 12, Google is doing nothing there anymore.
Opensource SIP Clients that can do more interesting codecs (think “HD Voice” - Speex/Opus) and transport encryption, look to Linphone. The excellent “Conversations” as app can do SIP too, though does it via xmpp (jmp.chat for example).
Closed source Apps with adjacent commercial sevice and probably easy registration and setup flow are Zoiper and Viber.