If the phone costs £140 down, and you must pay £15/mo, then to figure this out, we must simply to £15 x 12, because there's 12 months in a year, then add the other 140.
15 x 12 = 180 + 140 = 320.
So there you are, you pay in total £320 at the end of the year.
Hope this helps! :)