why my credit is being taken when I'm on £10 O2 big bundle ?

[Anonymous]

I just changed my tariff from o2 fav place/postcode to:

£10 Big Bundle tariff includes 100 minutes, 1000 texts and 500MB of 2G/3G data in the UK for £10 a month

After I  have sent my 1st text message I got pop up note showing me that  charge for that text has been taken out of my 10 quid ..i sent another text and money taken again.

Shouldn't i retain my 10 credit until i use all my free tariff allowance? 

Or maybe this is normal for that 10 quid credit to be used.....but then 10 quid would not cover the whole tariff allowance.

Can someone please explain how does it works?

When my credit should start to be spent? I always thought that It would be left untouched until tariff expires.

By the way i only use wifi. My data is always switched off.

[Anonymous]

Only Three seems to be offering PAYG in the traditional sense nowadays. Most other networks are encouraging customers to top up regularly to get access to the best deals. That's why out of allowance charges tend to be quite high.

[MI5]

They are out there, but they are all different across networks and MVNO's