I'm going to refuse to answer the phone to my mum after 7pm when I leave home
Why would you want to ignore your mom?
I'm tired of being disturbed by her
I get that you need your space and you are busy. I just think that she is concerned for you and wants to make sure you are okay. I think that you should still answer but just make it a quick conversation.
I don't care, I'm 18 now. Nobody tells me what to do, I get to make my own decisions.
You are an adult, and you have a point, but that doesn't mean you should  should no longer treat your mom with love and respect. You should explain to her how you feel, maybe let her know that if she's really worried, it's OK to occasionally make a quick call once in a while but not expect to have a long conversation.