In our case, my grandmother always calls my parents home phone number. The phone provider only offers simple call forwarding, so we route all calls to the home phone over to a Twilio number. We have a very simple Twilio Studio flow that routes calls from my grandmother's phone number to the call handler script we created, and all other calls are routed to my dad's cell phone number: https://share.cleanshot.com/ywwhzJ8H

We are a little lucky in that we can forward calls from our landline to a cell phone. If your relative is calling you directly on your cell phone, your carrier would need to offer selective forwarding for this strategy to work