I know you didn't ask this question. I urge you to not do this unless his mother is OK with it. By giving him a phone you are putting him in the middle between you and his mother. You are also teaching him to fudge on court orders.
I strongly suggest that getting him a phone will backfire in the worst way. His mother will know and will have the right to go back to court. The judge will not like the way you are subverting his order and may reduce your contact even more.
Even if she doesn't go back to court you are creating more negative feelings which will make it even more difficult for you and your ex to get along. Getting along is essential for the well being of your son.