I’ve used CSipSimple in the past and it worked well. But it’s removed from F-droid. I’ve tried the other SIP clients what I found on F-droid, but they did not work well: Sipdroid, Lumicall, Linphone.
It’s not possible with Linphone to create a new account. Maybe it’s no problem with an older version?
Sipdroid is for me not trusteable enough, often the phone does not ring.
Lumicall: seems to be possible to use Sip, but very complex to setup. After setup with help of my SIP provider it did not work. And it seems to be not very stable.
What are you using with succes?
Paul van der Vlis