Mobile phone under/over charge

Mrs_Pilgrim

Hi All,

My mum has had on-off issues with her mobile phone billing - sometimes they're over charged and sometimes they're under charged. I think it might be slightly in her favour now but I was wondering what time-frame the company has in which to realise their mistakes and therefore charge the correct fees? I've told my mum to try and keep track of it and keep the money aside if she thinks shes been undercharged but at what point does she stop this (as she's struggling to work out what it should be). Is there a time limit that mobile phone providers have in which they can no longer go and bill for old charges? I've looked at Ofcom but can't ascertain an answer (I think mobile phones are different to landline phones?)
All help will be gratefully received.
(P.S. I will add that she did report it some time ago and that's when it started going the opposite way and started undercharging which is great in one respect but not if they suddenly ask for a payment all in one go!).