She texted me and said, don't call me again. I will call you when I am free. What does this mean?

There are two possibilities. The first one is that he doesn't want you to be known by his family. He may have a conservative family and doesn't want him to fall in love or anything. In this case, it's easy to expose the fact that you two are together by phone. Another situation is that she has a boyfriend, and he is cheating with you. She doesn't want her boyfriend to know that you are together, so she will let you call.