What about writing her a letter? Telling her how much her friendship means to you, how you think about her and worry about her, and how you will always be there for her if she ever needs your help - she only need ask you. You can say that you understand there are times that she wants to be on her own, and that maybe she finds it difficult to pick up the phone. Tell her that you won't pester her, but that you don't want her to feel you are not thinking of her if you don't call. Then just send a text occasionally, asking how she is and telling her that you are thinking of her.
Letters are completely underrated in my opinion, and are lovely things to be treasured (well, not the ones from the bank manager, obviously, but you know what I mean)
I suffer from depression and like others have said, when I'm really bad, I can't answer the phone to anyone, I don't want to see anyone, talk to anyone, I can't possibly listen to anyone else's problems, I have no empathy, I am just drowning, alone, in a pool of black treacle. I have friends whom I love dearly and I have had to explain that this is how it is when I'm depressed. It doesn't mean that I don't value our friendship, I do. It's just that when that black dog sits at your feet, there aint anything anyone can do about it, except wait for it to go again.
(Sorry, mixed metaphors there)