I think this has been posted before (over a month ago) but basically its a cheap way to get this phone.
It's available on o2 for £20 month for 18 months of which you can downgrade after 9 months.
It works out to:
9 months @ £20 = £180
9 months @ £15 = £135
Total = £315
On top of that you can get cashback via Quidco/Topcashback so potentially this could cost £280.
The tariff is 100 minutes & unlimited text, no data.
However this deal is about getting the phone cheaply & not about the tariff.
This deal is probably not suitable if you want to actually use the supplied sim with this phone unless you add a data bolt on.
The phone is probably unbranded & unlocked.