I agree with @DaveGG.
Based on this reverse engineering of the A harness:
And the pin outs provided by @DaveGG:
And also the photo in your earlier posting:
is it possible that A1 should be connected to c and NOT b, which is as per the Ring wiring diagram (below)?
And maybe remove the link between 6.1 and b, or move the 6.1 link from b to c?
My A1 (BPT System 200 intercom with AGT A200K handset) is definitely connected to Common (aka Ground) but b in your system (based on the above pin out) appears to be supplying a voltage to the Chime and so doesn’t appear to be Ground, which could cause all manner of problems.