When power was restored this likely caused the phone to re-provision. You can look at the phone web interface and check to see if the provisioning path is set and when it is set to update. Most of the time it will be upon power-up.
If you manually provisioned the expansion module, then 3CX does not have a configuration for same and when it rebooted, and assuming that the provisioning path is present, then the phone reloaded the configuration as 3CX knows it to be. This wiped out the EXP 40.
You need to either turn provisioning off in the phone or provision the keys via 3CX; the latter possibly being the more prudent.....but perhaps not able to take full advantage of labeling.
