I have a prepaid mobile subscription and I want to know how much I phoned between two payments. There are plenty of applications that can track my phone usage in general, but I am missing the feature to specify which days I paid my bills. Is there any such thing?
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I couldn't find such a feature shipping with Android itself -- but: Yes, there is. Or rather: there are. Plenty. It would become a long list. Watch out on the Playstore for DroidStats or Call Meter 3G -- both apps allow you to specify the "payment day" (DroidStats is probably easier to use, while Call Meter can cover more complex scenarios).
There are also others like e.g. PhoneUsage, and more (you can follow the related apps for more suggestions).