You could use a computer with a headset as a linked device, that's the closest I can think of.

You can bridge it through e.g. Twilio but it will add latency that makes the voice calls less pleasant. You're better off with a phone that has a microphone kill switch, or physically remove the microphones (hack the hardware) and only use an external mic. Or power down the phone altogether.

Interesting concept. There's https://github.com/AsamK/signal-cli which specifically allows piping messages from Signal to another process, but at a glance its not clear if it allows for that with audio calls.

This would be a cool gimmick

If you want to maximize paranoia, don't forwarded end to end encrypted communication over unencrypted systems

As somebody else mentioned, using a computer and just taking calls there would work and give you solid control over microphone activation etc. If you really want the landline experience, look into adding a USB handset to that setup. It’ll just act like a mic and headphone from your computer but in the classic phone form factor

Haven't had landlines in a long time but when I did I remember looking at cordless phones with Bluetooth to connect to your cell phone so you can answer both phones (cell phone and landline) on the handset. Beyond that look into pbx.