Error when trying to use call forwarding

Also having this problem since around the same time. (last successfully forwarded call was 5/21/2024)

Phone: Google Pixel 7a

Briefly worked (about 3 weeks or so) on this phone, but stopped

Was initially setup by Robokiller app, but that can no longer adjust the call-forwarding, either.

Following T-Mobiles troubleshooting is unhelpful, since it doesnt actually talk about this error  (or any potential errors, for that matter) (https://www.t-mobile.com/support/tutorials/device/google/pixel-7a/topic/calling-amp-contacts/redirect-calls)

Additional threads tried:
 

https://www.reddit.com/r/GooglePixel/comments/14o8npa/call_forwarding_on_pixel_7_w_tmobile/

https://www.reddit.com/r/tmobile/comments/14oprkc/pixel_7_with_tmobile_esim_wont_allow_call/