On FreePBX 13.0.42, asterisk 11.19.0 - FreePBX in device and user mode. This is intermittent, but has happened to me with about 2 out of 10 extensions I set up. Here's what's happening.
I created a user with voicemail, then create a device as fixed and linked to the new user. When FreePBX configs are applied, the endpoint will work, voicemail will work, but asterisk complains with a notice on the cli console that it has "received a subscribe for a peer with no mailbox..." If I check the /etc/asterisk/sip_additional.conf, there is no mailbox= line for the sip extension. If I switch FreePBX back to extension mode and simply open the extension for editing and submit/apply, the mailbox= setting will be there and the notices on the asterisk console CLI go away. Is this by design? I would think the mailbox option would only be missing if the device was set up as adhoc and not fixed?