Settings > Date and Time > Time Format

Time Format
This setting specifies the format in which time is depicted on the display.
HH.MM: Selects the time format in the order of hour, minute separated by a point.
HH:MM: Selects the time format in the order of hour, minute separated by a colon.
 
Proceed as follows:
Using COMfortel Set (page Settings/functions > Display, list field Time format)
Using the telephone (menu settings > display > time format)

COMfortel 1200 - Firmware V2.2 - COMfortel Set V3.16 - Advanced Information V07 02/2019