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My friend took out a monthly phone subscription and now can't afford it, I am happy to take over it as she already took out a handset for me. I would be paying both handset subscriptions. How do I transfer these over to myself so I can help my credit file which isn't good? Thanks!
Please see here: Transferring your Number and Agreement
The form will need to be completed by your friend as the account holder, you will need to accept the contracts into your name. The one thing to consider is this will be subject to meeting Vodafone's credit check criteria and you mention that this is not good.
The other alternative is to split the account, your friend will remain the account holder and responsible for making sure the payments are kept up to date but will give you control over the account and payments.
Also, Vodafone have a three month clear payment rule and you may have a problem taking over two handset contract at the same time.
Ask your friend to speak to Vodafone through the Customer Service channels they will be able to advise accordingly.
Edited to answer reply from @spi3rzy
Before being able to take out an additional contract Vodafone have a three month clear payment rule on the first contract by which time they will have an indication as to how you manage the account.