Zen use WLR3 (i.e. BT) for voice. For broadband, they use their own backhaul network if possible, otherwise they use BT Wholesale.
Their preferred position is bundled voice and broadband, but if you have WLR3 voice from another provider (such as BT Retail or Pulse 8), they will sell just broadband if you wish.
If they are taking control of the voice element, there is a mandatory delay to allow a period for you to object to the move to Zen as a precaution against slamming. It may well be that they've chosen to delay the provision of broadband until they have control of the voice element of your line.