Phone Systems - two POTS lines into one Phone?

Does anyone know if there are circuits or products that will allow for connecting two separate phone lines into a single “phone”?

I’m working with a system that currently has a tip/ring wire as an input, the system auto answers this line and amplified that audio over a speaker string.

We are trying to make a secondary input into the system from another phone line. We attempted to just connect the second line’s tip/ring into the system in parallel with the existing tip/ring but things did not work.

Is there a product or a circuit out there that will allow 2 POTS lines into one?