L.M.
My daughters started using the phone when they were 2 to call relatives (aunt and grandparents). I think they started calling friends around age 5 or 6. Since they didn't have much to talk about with friends at that age, I really didn't need to set any rules as with time limits.
As they got older, the only restriction I set, which was the same as with their friends, no calls before 9:00am or after 8:00pm.