So... put the "extra" payment into a sock drawer each month until you have enough to pay off the amount and then your lease agreement is over. The agreement is until the phone is paid off or 20/24 months, whichever comes first.
Basically, everyone should do his/her own cost analysis. For me, with 2 phones to upgrade, buying outright and buying NEXT and then keeping the phones for 2 years end up exactly the same cost. Buying with a contract and keeping the phone for 2 years is more.
----------
I'm not totally following you, but IME, the 25% off does not apply to the per line cost. That discount applies only to the mobile share plan. So, for me, with a university discount, I get 20% off of the $100 for the mobile share plan. Then the phones are X per month (either $15 or $25 depending on whether on or off contract). No discount is applied to the phone line costs.