Adding on to what Grouchy already said (good post) :

• Like they said, safer for what? Against who? A landline can’t get stolen from your pocket. A landline can’t use encrypted E2EE messaging apps like Jitsi or Signal. Words like ‘safe’, ‘private’ and ‘secure’ mean nothing without context. They describe situations.
• I use a phone for far more than calls, a landline wouldn’t change that
• Most of the benefits of a security-focused ROM are against hacking and untrusted apps. These don’t affect landline phones.