The whole point of provisioning phones is to eliminate the difficulties associated with manually configuring a large number of devices: It is a tedious task It can be very time consuming Possibility of Human Error Retaining a Standard Config Enterprise-Wide Time, Time Zone, DST Settings The goal here is to get all phones to display the correct Time, including the correct adjustment for Time Zone and DST. We can configure all the phones to get the same settings by setting the necessary Custom Parameters to the correct values

See the article here:
3CX Phone Provisioning – A Complete Guide