Adding BLF to softphone also adds to the management console - thus a reboot of the YL phone should pickup the BLF change.
(On my Grandstream it did not pick it up on its own. )
As mentioned the users will need to do this from the 3CX clients. This will also add the BLF key to the management console and once the phone re-provisions it will pickup the BLF key. This requires Pro or Enterprise licence. https://www.3cx.com/ordering/pricing/