This report is incorrect (and so is the Wikipedia entry on the case). The correct time is 12:10pm (see
here) and it is known it rang twice (but was unanswered) that morning when her work manager tried to contact her.
Also they could not tell if it was turned off or ran out battery, but it was assumed the former probably because they thought she left as normal in the morning and would have made sure it was charged overnight. She had no land line or computer so relied heavily on her mobile and was and avid caller and texter by all accounts, so she would have ensured it was kept charged.
Now they are focusing on the previous evening it is possible it was not charged up, so it could have run out of charge the following day.