"을 테니까" and "을 테니" are almost same. for example, "집에 있을 테니까, 언제든 연락해" and "집에 있을 테니, 언제든 연락해" are same meaning that "I will stay home, so, call me whenever on today." and, if we need to find just difference, you can use just little bit nuance(뉘앙스). "집에 있을 테니까, 다른 친구랑 영화보러 가." => "I hope and want to stay home, please go to the movie with other friends.". "까" seems to make a nuance as a reason or my wish and feeling. BUT, anyway, they are almost same.