Unless you are a Now TV Combo customer, there is no phone number for Now TV as everything is done online.
Do you not have a credit / debit card linked to your Now TV account ? I was under the impression that you had to have this, so payments can be taken that way?
Alternatively, you can purchase Now TV Passes from places such as Argos / Currys / Game etc. and you apply them to your account, but you still need a credit / debit card linked to your Now TV account.