The other gentleman couldn’t answer better. Maybe I miss understand your question in first place.
You should have an FXO type of gateway to interface with the PSTN. This equipment will answer the phone and redirect the call to the final destination, that can be an FXS box like ATA 186.
I don’t know what is the size of your project but you can use a box with 2 FXO ports from Audiocodes for example or a Cisco AS5xxx for a larger operation.
Can you give more details of your project?