Just a note that on our v14sp3 deployment, this did not work as expected. Our Yealink T4x phones were provisioned to pull the DST dates from the settings on the server. I don't think these are sent out anymore with v14. I had to manually edit the Yealink provisioning template and reset/reprovision the phones. I did confirm that the 3CX server was in the correct time zone and was displaying the correct time.
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#Configure the daylight saving time feature; 0-Disabled, 1-Enabled, 2-Automatic (default);
local_time.summer_time = 2
#Configure the DST type when the DST feature is enabled; 0-By Date (default), 1-By Week;
local_time.dst_time_type = 1
#Configure the start time of DST. The default value is 1/1/0.
#If the DST type is configured as By Date, the value format is Month/Day/Hour. For example, the value 5/20/10 means the start time is at 10:00 on May 20.
#If the DST type is configured as By Week, the value format is Month/Day of Week/Day of Week Last in Month/Hour of Day.
#For example, the value 1/4/2/5 means the start time is at 5 o'clock on Tuesday of the 4th week in January.
local_time.start_time =3/2/7/2
#Configure the end time of DST. The default value is 12/31/23. The value format is the same to the start time.
local_time.end_time =11/1/7/2
#Configure the offset time (in seconds). It ranges from -300 to 300, the default value is 60.
local_time.offset_time = 60
#Configure the time format; 0-12 Hour, 1-24 Hour (default);
local_time.time_format = 0