I’ve been using Sipnetic for years, connecting to my asterisk server running on a VPS. For the most part, it works well. It is a little heavy on battery usage, but that’s to be expected since there’s no push option. I also use it for SMS and direct messages.
One oddity, is that I have to specify a custom DNS server, otherwise it won’t work. I am using wireguard on my phone, connected to the same VPS that asterisk is running on. It might have something to do with that; I’m not sure. But nothing else on my phone has a DNS problem.