TLS downgrade attacks are a thing, and can enable MITM attacks. There are server-side mitigations (such as only allowing TLS 1.2+ which should be the case but often isn't because the server has to support a niche user or application that only supports TLS 1.1), and since you usually don't know which TLS version you are using, for very sensitive connections it should be assumed that TLS is not enough.
Don't even get me started on the non-security of standard mobile/landline calls. They're basically transparent for an attacker with means like Russia's.
Proper E2E encryption and/or a VPN should be mandatory for a call to be considered secure, period.