I’d start communicating via email or text so you can answer at your choice of time, and have a paper trail.
If she’s the one initiating the calls, let her know texts/emails work better.
If you have to be on the phone with her, first thing say “I have 10 minutes to talk. Then I have to get off the phone.” At 5 minutes remind her you’ve got 5 more minutes to give her. At 10 minutes tell her you have to go, and that if she has anything else to say, to text you.
If she asks what you have to do, tell her it’s private. That starts to set the boundaries that your life is no longer something she is part of.
If she starts rambling in texts or emails, just respond to the business stuff that has to be addressed. Don’t encourage conversation by asking questions.
She’ll catch on.
If she starts in with the “don’t you want to talk?”, bite the bullet and let her know you only have time to talk about what is necessary.
You get to set the parameters, not her, and you aren’t her hostage.