General question - Busy/Congested messages in logs

Asterisk v11.6-cert1
FreePBX v.

We had an issue earlier where all four Aastra phones in a branch office wouldn’t ring when calling those extensions. Calls to them would go straight to voicemail. I ultimately resolved this by running ‘core reload’ at the CLI.

I saw this message in the /var/log/asterisk/full log:

[Mar 25 09:23:22] VERBOSE[15290][C-000061e5] app_dial.c: == Everyone is busy/congested at this time (1:0/0/1)

I didn’t see any other errors before or after the above message. The call that the above message was from went through to that user’s mobile phone via followme, so the call wasn’t hung up or anything.

My question is, what are the general causes of the busy/congested messages?

You should have done a sip show peers. My guess is your users are not on the same LAN and the remote firewalls are not being sent “keep alive” messages from the phones.