Not disconnecting the landline will not physically stop your new provider giving you a line with a new number. If you are transferring your existing number, then because Virgin does not work through BT telephone exchanges it takes a while longer for numbers to be ported to other providers.
Also be warned that by cancelling your Virgin landline account it puts a disconnect notice on your number. This will effectively block the transfer from going ahead & you could lose your number as a result.
Cable customer since 1993. Services: FH TV, Sky Sports & Movies (2xV6), Talk Anytime Phone, VIVID 350, Virgin PAYG Mobile