I assume you are up to the E4/Clair/the trainers in-between... use the exp share if you have none yet (trade the red scale to Mr Pokemon for one) as that might help. As for areas, either below Blackthorn or victory road's wild Pokemon is the best in that regard, and you can rebattle trainers via the phone as well.
Here's when you can call who - note you can battle them as many times as you like in their time period.
Ask questions in the Help thread sticky next time please which is at the top of this section. Closed.