You are very close! You could say "Call me in the evening, because in the afternoon I'll still be studying." Alternatively, you could say, "Please only call me in the evening, because I'll be busy studying all afternoon." It is more common to excuse yourself with "I'm busy", and then explain the specific thing that you will be busy with afterwards. "Only" is a very useful word here, as it says "don't call at any other time" but doesn't require the reader to call if they don't have a reason to call.