Hi Ixel,
From a practical viewpoint there is no need for any phone service on a VDSL line via a FTTC, as the then Rutland Telecom demonstrated, despite the major obfuscation they encountered.
However I believe it is a BT wholesale stipulation that a phone service is mandatory so that is almost certainly included within the terms & conditions you agreed to.
Given that several ISPs now offer phone and VDSL broadband services combined or individually, I believe you could in theory make any changes you wish, subject to any contract term you have agreed to.
Kind regards,
Walter