Paid off iPad but extra charges added on?

[Mila27x]

I paid off the rest of my sky mobile bill to basically pay off an iPad. I paid the remaining amount the beginning of the year.. but I have now been sent a letter to say I still owe £144 towards the sky mobile bill and it won't let me make changes to my broadband or upgrade it until it's been paid. Ive tried to check my bill but it shows nothing of me paying it off on the bill. I have no way to be able to call them at the moment so I'm so stuck. I would like to add that the iPad doesn't have a SIM card etc.. so there's no way of me using data, it's only ever been linked to wifi and can't connect outside wifi. Any help would be great. 

[caesarome]

Are you sure you made this payment towards paying off the iPad and didn't in error make this payment towards your account. One way to check is to see if there is a credit showing on your account.