I use this.

I have Ms VoIP as my provider. $1 month per line and 1 cent per min.

I have a grand stream sip adapter... Configured with my providers info and my creds. Phone can plug into it. Works just fine. Plus you can use any sip client to connect to the same line from mobile or PC.

Also, my old house had pots lines in every room. I took my grand stream and wired it into the house, not a phone. So I can plug regular phones into any of the original outlets and it works as it directly connected to the grand stream. I have 4 regular phones all running over original wires into the grandstream using sip

Just use a sip phone app on your mobile. I use Linphone but there are lots of them.

f-droid.org/en/packages/com.tutpro.baresip.plus/

You might need to set a STUN server or similar, in settings.

Save yourself a lot of trouble and get a hardware SIP phone like the Grandstream WP826. I spent years struggling with software phones, most of them suck ass and the good ones aren't very good.

quick unboxing video

Then make sure to enable voice transport encryption and set SIP transport to TLS (not UDP) and set keepalive timers to something below 3 minutes or so. The encryption settings are not just for security, using TLS for SIP transport has way fewer problems with incoming calls than UDP in 2025 network environments.

There are some firmware issues with the Grandstream WP826 but they are steadily releasing new firmware updates every month or two. If you need absolute bulletproof reliability go for something more expensive, otherwise the WP826 or similar model will likely be good enough (I say that as someone who is easily vexed by shitty software/electronics)

I do this with avm fritz box, it can take the physical line and act as a sip server on the lan. I connect via the integrated wireguard VPN and use portsip app for make phone calls

You can install Linphone from the Fdroid repository, and register it with your SIP provider as an extension. How you set it up as an extension will vary by the provider.