There are a number of providers that offer such a service with the SIP protocol. The upside is that you're not tied to a single provider; the downside is that

There are a number of providers that offer such a service with the SIP protocol. The upside is that you're not tied to a single provider; the downside is that most providers only offer unencrypted calls (using RTP, not SRTP). OVH, for example, offers unlimited calls to landlines in 40 countries for 1€ per month: <https://www.ovhcloud.com/fr/phone/voip/ https://www.ovhcloud.com/fr/phone/voip/>. Now that Android no longer includes a built-in SIP client, you must use a third-party application. I've had success with Linphone (ignore their somewhat obnoxious on-boarding procedure), I've also tried Baresip, but I've had it hang on occasion. Both are in F-Droid.