Ok, so Avast developers have helpfully been looking at this (with only a tiny bit of input from me as I can’t reproduce it after uninstalling and reinstalling).

Here’s a summary of what I’ll do from now on to (hopefully) ensure that this doesn’t happen to me again:

1. Look out for “weird” entries in the blacklist.
In particular at one time I had “+0845xxxxxxx” (basically a UK non-geographic number) in my blacklist, but how the “+” got there is a mystery (suspicion is that odd entries arrived there after accepting the pop-up offer to block a frequent caller). Even more strange I also had the simple entry “+0” which shouldn’t have been seen as a wildcard (and wasn’t at one time, it had been there for many months without problem) but this was identified as the most likely cause of sending all calls to voicemail. And a reason I can’t now reproduce the problem is that I am disallowed from adding such a silly entry manually to the blacklist.

2. Update to Avast 6.5.3 or later.
This has several improvements to the call blocker, including stopping crazy numbers being automatically added to the blacklist !