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That may be owing to the configuration of the lines/ports on your phone server box.
Try switching two phones, one that has a date/time display, and one that doesn't, from the extension of one to the extension or line of the other... and vice-versa for the other.
See if, then, the problem
- A) follows the phones, in which case it is not a phone server configuration problem
- B) stays with the lines, in which case it is not a problem in the phones, but in the server box.