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If the mobile number is in your dad`s name then only he can sign up / recontract for a new handset as he is the account holder.
If you are looking to change the number in your name then a change of ownership request has to be submitted either online → here (can take 10 working days to process) or you can do this at the shop as well (then and there). However both the current account holder (your dad) and new account holder (you) have to be present. More info on change of ownership can be found → here.
Once the service is in your name then you can sign up / recontract without the need of your father to get the new handset as you will be the account holder.