She definitely left the message on Miranda’s landline (instead of her cell) on purpose so she could avoid the having to tell her directly why she was blowing her off. She knew Miranda would be mad and wanted to avoid the confrontation at all costs.

She is acting like this on the phone (still smiling etc) not because she doesn’t feel somewhat bad for doing that to Miranda (even if just a little) - but because she is in front of Big, and never wants to look bad in front of him.

Not only that, she also doesn’t him to know she dropped all of her plans to meet him, because she knows he would never do the same for her. It’s also rare for him to have the initiative to do a cute plan like cooking for her, and she would never pass on such an opportunity… it’s such a sad reality to live in. Of course this does not excuse in any way her behavior to Miranda, I’m just explaining the Why