I agree with GSDs4Ever
It does help to set limits up front. When she rings - tell her you can listen for 10 minutes and then you have to go, try not to give a reason but if you do - give something that has a limit ie the cake will be done. (if you cook cakes).
And use a timer, and when ten minutes are up, talk over the top "gotta go" and click. She will eventually learn to focus on what's important. And occasionally you can reward her with what she clearly wants - ie talk away darling I've got all day today.
I suspect training MIL is like training puppy - reward behaviour you want and ignore/deflect behaviour you don't. Also have you tried putting a grandchild on the phone when she's had her ten minutes. Or would that make no difference to her words/behaviour.
Also when you do decide to "have the talk" - write down all the things you want to say in point form before you meet face to face so she can't frazzle you into forgetting to say what's important. And it's definitely a good idea to tell her what you need, even it then takes her weeks to process and accept it.