I have now also added an entry in /etc/asterisk/sip_notify_custom.conf (didn’t know the notify event type was in /etc/asterisk/sip_notify_additional.conf, thought it was somewhere in php code, thanks for letting me know!
I have to correct myself in my above statement where I am saying that event “check-sync” would not reboot the phone, provided that sip.notify_reboot_enable=0 is set in basefiles. I got this from the Yealink provisioning guide but it’s apparently not true:
I tried “check-sync” only but the phone rebooted whether sip.notify_reboot_enable was set to “0” or default “1”.
I tried check-sync;reboot=false and this works regardless of the sip.notify_reboot_enable value, the phone takes the config without rebooting. I so far only tried changing a line key, but will try more.
Anyhow, looks like I got the phone to behave the way I want, thanks @necits2015, good teamwork, I will keep you posted if I run into any other issues.