Square Enix has been adamant about getting a lot of the big titles from their past games up on smartphones as well as original games based off of those classic franchises. This is especially true for the Dragon Quest franchise as they have plans to release every title in that series on mobile platforms and they announced recently that their MMO, Dragon Quest X, would be the next title to take the dive into the smartphone.
How does it work exactly? You need to have an active subscription for the actual game, either the Wii, Wii U, or PC version. Then you have to pay additional for the phone version. The pricing is set at 350 yen for 3 days or 650 yen for 10 days and this is obviously in addition to what you are already paying for the main version of the game.
The game just launched today in Japan only so I don’t have a hands on impression as to how this is going to work but it appears to be a compliment to the main game as opposed to being another way to play it entirely.