If you don't want to do as MrTango suggests, then my suggestion is to respond to her request with a 'no, that won't be possible and I have to go, have a good day, goodbye' followed by you hanging up. You told her your hanging up, so you aren't hanging up on her, but just because she wants to keep talking doesn't mean you need to keep listening.
We are dealing with boundaries with my kids - they are young, but the pushing of them is making me nuts and the counselor we work with told me that in his home, his kids know that after they ask 2 times, he just doesn't respond. He doesn't say yes, he doesn't say no, he doesn't say I've already answered that, he just doesn't respond and they know to stop asking him after two times (his wife doesn't do that, and they bother her over and over and over again). I'm trying to do that with my kids, and I think it may help with your SIL.