The Sky+ contract states that a new box must be connected to a phone for at least 12 months, because of the subsidy. Boxes are triggered by satellite to make calls back to sky to confirm they are connected. Some people get away with disconnecting their digiboxes almost immediately and never hear anything from Sky at all.
Sky+ boxes are supplied with recording disabled, and I believe they will not enable it until they confirm a telephone connection.
A second hand Sky box can be used without a telephone connection, and a Sky+ box can be used to replace a digibox, but the Record facility will be disabled. Sky charge for the recording facility - £10pm. The charge is waived if your package is £30+.
So you could buy a second hand box and get it enabled with your current card - then phone up and amend your contract to pay the rest.