My 'One Plan' contract ended in Feb this year. Three called and then Text to get me to contact them, eventually when I continued to ignore them they sent a text with the new plan i would be put on when my contract expired.
I did ring them at that point, as I wasnt happy with the lower call limits ets, and after several days and calls, agreed an unlimited calls, test and data for £15 (still with a theoretical 4GB limit on tethering, - tho I've yet to see that happen).
I was a high data user, and went for the EE 15GB sim, same cost as Three - BUT they will allow a 30 day rolling contract - Three wanted a 24 month fixed contract for a sim only data plan. - stupid!