Location: You need to complete all of Barry's mission for Franklin, i.e. Grass Roots- Franklin, Grass Roots - The Drag and Grass Roots - The Pickup. Wait for the marker to appear in the Alta district, which will let you know that you can talk to Barry again. After you get there(the region of the town hall) you will not meet Barry, but you will be able to have a phone talk with him.
Rewards for the completion of the mission: none
Requirements for the completion of the mission in 100%: There are no additional requirements for this mission so, it gets completed in 100% automatically.