SBC Phones can only register to the system if they are running the version of firmware that matches the version loaded on the system.
Because of this, it's now advisable to onboard all of the phones before downloading the updated phone firmware. The system should still be patched onto the latest available release, but WITHOUT selecting to download the newer available phone firmware.
You can see if there are any available firmware updates from the updates screen. This will tell you the version currently installed.
If there are no updates showing, then the system has already downloaded the latest firmware, so the version can be viewed here: https://www.3cx.com/docs/phone-firmwares/
For example:
Server version - 96.86.0.81
Phone Version - 96.86.0.81
SBC phone will register without issue to the server.
vs
Server version - 96.86.0.81
Phone version - 96.86.0.70
SBC phone will not register to the system.
In the bottom example you will need to download the latest firmware from the 3CX site, log in to the phone's web interface, and manually apply the update.
Once updated the phone will register and be able to receive further updates through the web console.