1. She will call you if I have time.
2. She will call you when I have time.
What are the differences between the above sentences?
May I try:
#2 is more certain that I will call rightaway as I have time.
#1 the possibility of calliing back is very low even she's free.
In answer to your question though, sentence 1 means that it is not certain that she will call at all. She will only be able to call if she has time. If she doesn't have time, she won't call.
Sentence 2 means that she will call but it's not certain what time she will call. She will call at some point during the day but only when she has enough time to make the call.