On Editor and on Android device it works fine but when I try it on iOS it just crash when receiving a callback.

Unity Version: 5.3.2f1
FMOD Version 1.07.06

See this post which is about a different callback type but the solution is still relevant to EVENT_CALLBACK. Also see http://www.fmod.org/documentation/#content/generated/engine_new_unity/script_example_timeline.html