I understand why you are picking up on ambivalence. But. She is okay with doing it. She offered it. It's okay for her to want you to call, especially if she is normally in at the office instead of being at home. She doesn't want to forget. But she prefers you to take it on you to call in at the right time. Just in case.
Honestly, I understand why you are reacting to it, I would feel guilty etc as well. But despite the oddness or the out-of-routineness of the arrangement, T is okay with having a phone session with you on Monday. Once you are on the phone together she is going to be with you and in for the session. It'll be okay, I think.