I'm still on the stock 7.1.1 ROM and I can not see any SIP settings on my phone.
However I'm using an app called Zoiper on my Nokia 8 and it works fine with my SIP provider for making and receiving calls
Since I got my new Nokia 8 now, I can prove that SIP accounts are available.
Open Phone app and go to Settings --> Accounts --> SIP-Accounts
What about sound quality? I'm on 7+ and the SIP voice call quality is horribly choppy =(