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.

This seems neat. I have some minor stuff I'd like an ATA to do differently [1], and maybe I can make it work with this, if I get motivated!

[1] I don't quite remember what I wanted to do, so that's the first part! But I have a telephone controlled gate, with 3-pair wiring, and right now I'm using two pair for phone inbound and outbound, and one pair I can't use... If I can get the ATA to interface well enough with just one pair to the gate, then I can use two pair for ethernet, and I don't need to use a modem to interface with the gate over the phoneline.

I cant remember the circumstances, it might have involved an RMA with grandstream, but I vaguely recall this tool saving my bacon at one point.