Nice project! Another route you might consider is flashing a small custom firmware on the Grandstream HT80x so the logic runs directly on the ATA instead of on the Pi/Asterisk side.

I wrote a tool that can extract and repack Grandstream firmware (https://github.com/BigNerd95/Grandstream-Firmware-HT802/tree...).

For example, I made a simple ARM patch that intercepts incoming caller IDs and triggers a bash script to decide what to do, in my case it checks a spam-lookup service and returns 0 or 1 so the ATA either rings or rejects the call.