is this permanent or temporary overseas?
Are you out of contract/at the end of the minimum term period with Now?
With most landline/broadband services in the UK there are at least two ways to cancel:
1: by calling the service provider
2: by porting the number out to another provider
If a UK number would be useful to you then you should be able to port your Now landline number that comes with your broadband account to a Voip service , in theory this means you will have access and be able to use the UK landline number anywhere , and via an app on a mobile phone or an adaptor ( or both) there is of course a cost associated with this either a small fee and then an ongoing cost, ie andrews arnold ( appear) to charge £1.44 to port a number then £1.44/month as well as call costs , others have free prting but a higher monthly fee with inclusive calls, or a higher port cost and no monthly fee but higher paay as you go costs
Porting the landline number to a Voip service will also have the effect of ceasing the broadband assocaited with that number and with it the cancellation of your Now broadband account
If you take what i have said and ask about it on a more technical forum then you may get a better solution, i would recomend the forums at ispreview.co.uk as there are some industry people in there who can offer more help
