(update: I think I realised that this only applies to PJSIP. With chan_sip it works the same as the rest of the Internet would tell you).
6 hrs later, I discovered that Cisco SPA5xx with firmware 7.6.1, and Asterisk 13, require the ‘system type’ in the attendant console settings, setting to Broadsoft. It doesn’t work when set to Asterisk (Directed Call Pickup doesn’t work). This has been a long night.
I also had to set the function as below:
without the ext= it still doesn’t work.
It needs both, sub= and ext=, and system type = broadsoft
So, broadsoft as system type, directed pickup code **# or whatever, disable lines 2 – 7 or whatever, set type to private, and use above extended function.
This applies whether you have an attendant console are just using the spare line keys.