Hi
Big talker Payg allowance usage is only to other people on the O2 Network.
From o2's Site "Big Talker is perfect if your friends and family are also on O2. Top up and get O2 Calls, O2 texts and data – use your credit to buy more when you need to."
If you send texts to people not on the O2 Network ,then I'd suggest you need Big Bundle. Payg Tariff instead.
Tariff info here ->
http://www.o2.co.uk/tariffs/payandgoTheir is a £10 and £20 Big Bundle Option.
Your only supposed to be able to change Tariff once a month ,but if you speak to Customer Services on 4445 and explain they maybe able to help on a one off basis to change your tariff over.
They may do this free of charge as a Goodwill Gesture (which they've done for me in the past) or you may be expected to top up again to change.
They will officially confirm which route they'll take on the call.
Hope this helps.
😄