Lost Pay as you go number due to lack of use.

[Wullie]

 Due to lack of use my old payg phone has been disconnected from the network. How can I get it recconnected?

[MI5 · Accepted solution]

@Wullie call or chat to customer service and if it hasn't been too long, they will reconnect you. 

http://www.o2.co.uk/contactus

[jonsie]

Call customer service or use live chat in the morning which can be accessed using IE or Edge browsers.

If it's not too long past the 6 month rule then they could reactivate the number and replace the credit.

[b4u2]

usually once your number is disconnected then you have lost it therefore always make sure to do at least one vchargeable activity on your account every six months

[MI5]

Providing the number has not been reallocated, O2 will always reconnect you.