Yealink phones need to reboot on restart of Freepbx, or else the BLF fields don’t work. I had this working before, but now they will not reboot automatically on restart. This was the old solution:
The phones need to be online and reachable by asterisk when notify is sent. If they aren’t, the notify will not have any effect. I would check this first.
If the phones are online, I would try sending that command normally through asterisk CLI and see if it reboots a phone. Not sure with Yealink, but some manufacturers will not reboot on a check-sync unless there is a configuration change that requires it.