The half-price broadband offer is a promotional offer for new customers signing up for broadband and phone. It isn't available to existing customers. However calling the Customer Options Team on 0800 432 0200 (0900 to 1730 Mon-Fri) to discuss your requirements might be worthwhile.
Plusnet offer Line Rental Saver for paying 12 months line rental in advance which is equivalent to £10.49 per month. This needs to be done online at
https://portal.plus.net/apps/linerental/saver/portal[quote=https://portal.plus.net/apps/linerental/saver/portal]
Line Rental Saver is the cheapest way to pay for your home phone line rental.Pay for 12 months line rental up front for £125.88, equivalent to £10.49 a month, compared to our usual line rental price of £13.99 a month.
David