As you all know, Android has disabled the built-in SIP service on Android 5.x and above.

Has anyone found an alternative for the SIP service to work?

glad to hear some thoughts.
I needed SIP once, I googled android SIP and had lots of app.
I've tried several one and all of them were ok.
But no app is better than the original built in sip for android because of the ease of use.
Try zoiper from google play
Thats what im using right now. But the built in service is much better