Call duration timers

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Hello,
As this is my 1st post I hope I've got it in the right place!
Q: Can anyone pls tell me what maintains the timers to be found in settings>device info>call duration
I ask (for my education) as I have just bought another o2 mini s from ebay to act as a backup for my existing phone. The seller said that the unit was 'hard reset' so was in 'as shop' condition. Indeed, it is perfect and whilst cloning it to match my everyday unit (both G4s BTW) I found that these timers are zeroed. Over the past few years I have hard reset my phone a couple of times when it lost the plot, but the call timers remained. I did not remove the battery so I am wondering if it is removed (as in the case of the new device) then after a while the backup battery goes 'flat' and the timers reset.
Best regards to you all.