Depends on why your mother said no. If she said no because of the budget, then talk to your mom. If your mother said no because she doesn't want you to have a phone for other reasons, no. And in that case it makes your boyfriend disrespectful of your mom.
It all depends on your age and why she's saying no. If she's ONLY saying no because your boyfriend got it for you take it. If not, then find out her reason, and think it over. Ridiculous reason: take it. Understandable reason: don't take it.