This balance (quantity not currency) is tracked in real time mode by the handset and the user is notified when it ends.

手机会实时记录更新该余额(指“存量”而非“货币”)，并在用完时提示用户。

youdao

But buyer beware, if you use the handset in this mode you cannot receive calls.

FORBES: Verizon's iPhone: 5 Things Enterprise Executives Should Consider

They need a UI ninja that will be able to direct the whole look and feel of the UI so that everything acts consistently in just one way no matter the mode or app that the handset is in at that moment in time.

FORBES: The BlackBerry Z10 Might Not Be A Stunning Smartphone, But It Gets The Job Done

The trio discovered that quickly connecting and disconnecting the battery of a frozen phone forced the handset into a vulnerable mode.

BBC: Frozen Android phones give up data secrets