This isn't something that can be spoken of lightly, so I won't give you any banal comments or pat answers - true, there is a legal aspect that means your mother would be in a lot of trouble should she abandon you and any siblings you have but the law does not prevent things from happening, it just deals with the aftermath...
Having said that, there isn't much info to go on in your post, but if your father left 2 months ago then I am guessing this might be the first time your mother has had to raise a family alone and I am guessing she is struggling. You could be the best kid in the world and it still wouldn't change her feelings about being on her own, trying to keep everything together, trying to figure out where to go from here.
But having said all that, I'll level with you here. Chances are, your mother said what she did to cause your father pain - to make him feel responsible if you and your siblings were abandoned. To make him feel bad for leaving you all in the first place - primarily, for leaving her. It wasn't possible for you to hear what your father was saying on the end of the phone, so you only got half of a conversation - and people will say horrible and desperate things to harm one who has hurt them.
The question you need to think about is, has your mother ever talked about leaving you alone before? Has she ever made any indication that she will abandon you? It's quite likely she may be depressed with what is happening and depressed people have very little perspective of their current situation and everything may seem totally out of control - however trivial it may be.
If you think it might help (and is possible) maybe talk to her about what she said? That probably isn't very feasible, but if you two have a close relationship it could be possible. Or maybe talk to your dad? The most important thing is not to panic because whilst parents say horrible things, it is rare that they will actually go through with what they say unless they are already that kind of person and if that's the case then you would have seen it long before now.
I don't know where you live, but is there any kind of free organisation for kids that you can call up and ask what they think? It would be in the strictest confidence and they are not in the habit of ringing the state to take you away, so you are safe to talk to them.
Just a couple of ideas. Hope it might help.
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