Unexpected and unwanted phone bill

Earlier this month:

Oxygen: Oxygen customer service, can I have your details blablabla.
Me: Blablabla details. Can I enquire how much credit I have left?
Oxygen: *Checks* You have 14 hours left.

*Cue phone call spree galore*

A few days later:

Oxygen: Oxygen customer service, can I have your details blablabla.
Me : Blablabla details. Can I enquire how much credit I have left?
Oxygen: *Checks* You have 38 minutes left. Until towards the end of the month.
Me : So little?
Oxygen: Your other minutes have probably expired by now.
Me : Hmm okay. *Makes mental note to only call if absolutely necessary*

This morning:

Me : Hmmm, private call? *dials*
Oxygen: You are at least 50 GBP above your monthly phone bill. Please wait while we transfer blabla



Oxygen: You have spent a whole fucking amount above your monthly bill
Me : That cannot be. Can you break it down for me?
Oxygen: Your two biggest bills are for two calls at 25 pounds and 15 pounds, to
number 07 **6 **1 **6.
Me : Impossible! I made two calls...
Oxygen: Well it says here...
Me : Don't interrupt me! I made two calls to your colleagues earlier this month.
I was advised on both occasions that I had 14 hours and 38 minutes remaining
on my credit. I do not see how I have dipped below zero. *temper flaring*
Oxygen: *Robot mode* I'm just telling you that you have spent....
Me : Just tell me the other calls.
Oxygen: Blablabla
Me : I clearly made those calls when I was aware of the time I had left. And
there is no way I would make calls when I have a second line, let alone long
lengthy ones!
Oxygen: Anyway I just want to know if you are going to pay for it.
Me : NO! I challenge that amount.
Oxygen: Then I will have to freeze the line
Me : You do that, I am too busy to argue with you right now (I was hurrying to
university). But I will definitely get back to you. *puts line down*

Arghhhhhh!!!