I would not be too hard on her, though. This is a huge change in roles and relationship, it is not easy for anybody, it is emotional, and this is a lot of "firsts" for her. It's kind of like being a mom with your first newborn baby - you may have some idea of what needs to happen, but you've just never done it before. There's a learning curve. Hope it works out well for both of you.
Bold above: I'm taking that to be DD being the new mom with a first newborn -- she
has a lot to learn about things she never did before.
Although it can also be taken that OP also has a lot to learn, which is true. OP, you've never cut the apron strings before so all of a sudden, life is quite different.
Yes, there is a huge learning curve for both of you.
I have adopted a very neutral tone when she starts whining and when she is done with whatever rant I just say "And what do you think you should do?".
Personally, I like what you're doing. I think you should keep a strong attitude and, yes, I do think you should cancel the cell phone.
I also think you could show a gentle side* . . . remind your DD things like "Hey, if you really
need me, I'll be there for you." and "Kiddo, you know I love you. You're going through a tough time. Guess what? Being an adult is tough!"
Tell her about some of your mistakes so that she doesn't see you as super-mom that automatically knows everything.
At the end of her rants, try to build her confidence "I know you can do it! You've done this that and the otherthing. You're already making progress!"
Even compliment her hair! Tell her that you never would have had the guts to do that when you were 18. (Yeah, okay, try not to giggle.)
Invite her out to dinner and/or a movie or shopping. You know, a girls get-together. Ask her about life in general . . . not what she needs, but what are her dreams? Tell her that when you were 18 you always dreamed about going skydiving (or whatever) . . .
She want's to be an adult. Respect her as one.
If, when you're out to lunch/dinner/whatever, she starts ranting on "you're so mean! You took my phone away!" Simply hold up your hand and say "I don't allow my friends to treat me that way." Then bean-dip and continue on with a friendly conversation.
*I'm just throwing out thoughts and ideas. I'm not a counselor nor do I watch a lot of TV, but sometimes being a parent means cutting the apron strings and allowing the child to learn to be themselves . . . even though you grit your teeth, sometimes you just have to let them know that you are proud of them.