Although Google stopped supporting the OS integrated SIP client and eventually removed it, you can install Acrobits Groundwire or Bria. Those support PUSH notif

Although Google stopped supporting the OS integrated SIP client and eventually removed it, you can install Acrobits Groundwire or Bria. Those support PUSH notification for incoming calls. Push is better than missing calls because the app got killed, or forcing the app to run 24/7 and severely shortening battery runtime. But the call quality will never be as good as the native phone app as that gets QCI prioritization.