This news is coming from the Kojima Station, in the latest episode Hideo Kojima said that the game is fully available to the players to enjoy the pleasure of free roaming and complete side activities, but once you start the mission, you will not be entertained with the Free Roaming and you have to stay in the specific mission area.
The Phantom Pain won’t permit Free Roam while missions
The missions, this time around will not be taken from the main menus, instead they will be a part of the open world of Metal Gear Solid V: The Phantom Pain. If we talk about the overview structure of the missions, it will be almost similar to the previous release, which is MGS: Ground Zeroes. You will be confined to a place when on the mission, but after you are done with it, you will be able to explore the open world.
At last Hideo Kojima said that, there is still a lot of work left on the Voice and we try over best to complete this task as soon as possible. If we talk about the release date of the game, still there is no specific release date. According to the previous leaked news the game is expected to be released in Q1 of 2015.
Source: Kojima Station