Nokia 5170i User Manual page 51

Setting timers back to 0
Use Menu 3 5 4 to clear call timers. You are required to
enter your security code.
Displaying the current call timer during calls
1 Press Menu 3 5 1.
2 Press Details.
3 Press Select.
4 Scroll to On, then press OK.
During a call, the amount of time spent in that call is displayed
for ten seconds.
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