I did some poking around and figured out that I could run something like
“asterisk -r -d -d -d” and get more detailed debugging info. That produced this:

[Dec 5 08:16:50] DEBUG[28802][C-0000000b] sig_analog.c: waitfordigit returned ‘
8’ (56), timeout = 0
[Dec 5 08:16:50] DEBUG[28802][C-0000000b] sig_analog.c: Can’t match 8 from
‘400
2’ in context from-internal

OK, so the fast busy comes as soon as asterisk can see that there is no extension that starts with ‘8’. The dahdi-channels.conf file does declare from-internal as the context for the channels those phones are connected to. And there is no from-internal context declared anywhere. So this makes sense.

What DOESN’T make sense is that this configuration *ever* worked. But it did, until the recent upgrade. The fix was to change “from-internal” to
“internal” in dahdi-channels.conf . So that just leaves the question of how this configuration ever worked at all.

–Greg