There was a thread on here a while ago where the OP had a friend with a son the same age as her own, and he had two little sibs. The older child spent so much time with the OP, and she was starting to become a surrogate parent, even to the point of monitoring his health condition. Friend rarely reciprocated hosting and rarely paid the OP back for money that was spent on her child. Then the other mom/supposed friend wanted the OP to watch the two younger children. OP said no. Friend pretended she didn't hear and ramped up the efforts. Then the friend started playing the 'poor me' card, and it became apparent that the friend had never looked into alternate child care arrangements; she assumed she could dump her kids on the OP. When the OP said no, the friend got worse. I'll try to look for the thread.
OP, get out now while you can. Perhaps you should go a little bit 'frustrated and at the breaking point' on her. When she calls yet another time: "I'm sorry, Friend, I can't do it." You can try to get away with a few reasons like you need a break, you have things to do, or tonight won't work out. Even ask her what happened to her 'regular babysitter' (even though you know full well she doesn't have one). But if she keeps pressing, then I vote to let her have it, politely of course. Sometimes that's the only way that you can get through to people is to let your frustration show. "Why can't I do it? Because I was supposed to help out in a pinch, for an emergency, and this is no longer an emergency. It's become a regular thing. It's like I'm one of your regular child care providers, and I didn't sign up for that! I work full time and I have my own family. I have to take care of them. I can't see my own family or do my own housework because it seems like I'm always at YOUR house, taking care of YOUR family. If I wanted another part time job, I'd get one. And frankly, I offered to help out, to be a friend, but now this has gone on so long, with no end in sight, that I just feel taken advantage of, and I'm stressed out. Honestly, I don't even feel like your friend anymore; I feel like you think I'm your servant or unpaid employee. I'm sure you don't mean to come across this way, but that's what it has turned into."
Honestly, she'll probably get huffy and won't speak to you for a while. But she's in for a rude awakening if she thinks this is how reasonable people find childcare. You could try not answering the phone, but it sounds like she'd probably just come to your door and ask why you're not answering your phone. So you'll probably just have to be blunt.