Explain bill pay to me?

My mum switched to bill pay from a pre pay phone, and chose a plan for £30 a month and got an iphone (so she's now on a contract, which is fine) but the staff in the shop gave her that much information at once she's a bit confused.
She got a bill for £1.40 - so does that mean she only pays £1.40 or 33.40?

She is paying £30 fro a plan that allows her to have so many free calls, so many texts and so much internet access/month
If she exceeds any of these allowances she pays extra

The £1.40 is the extra costs cos she went over her allowances during that month and its on top of the £30

(and I would VERY strongly advise she reads the phone manual about putting a password on it, so that this has to be entered before anybody can use it
(or maybe go back to the shop and ask them how to do it)
Without a password anybody can pick the phone up, call anybody they like and she will just get a bill for that call at the end of the month but know nothing about it
There have also been cases where people have picked the phone up then set it to dial premium numbers. When she dials a number, the phone dials the premium rate number first which then reroutes her calls to the number she actually wants.
Result is she calls numbers just as normal, but instead of normal call charges she get a HUGE bill cos they go via a premium number, Again SHE don't know any of this until she gets a HUGE bill at the end of the month)