i use sipgate as I said, costs nothing unless I use it.
I always worry that companies with thousands of users earning £0 won't be there when I need them. The costs of running the infrastructure aren't £0.
That's why I went with AAISP too, then found it has a super useful feature to automatically e-mail a copy of every call to you (as both of us in the household have awful memories).
Also the support, I found the COVID-19 helpline didn't work, contacted them and it was fixed really quickly.
We both have mobiles on PAYG which inevitably run out of credit at the worst moments, so having VoIP use top-ups too was a no go. Don't want it running out of credit after you've just spent an hour waiting on hold.