The 20-year-old rising star was born in England but he also qualifies to play for Scotland due to the fact that he has a Scots-born grandfather.
Phillips said in a number of Scottish newspapers: "I spoke to Stuart Pearce personally, because it had to come from me.
"A lot of people might have sat around and waited (for England). But I told him I wanted to come away and play for Scotland.
"I had a number of conversations with him but I haven't just made this decision in the past month. It's been an ongoing thing and something that's always been at the back of my mind that one day I'd play for Scotland.
"I believe enough in my ability to think that one day I would have played for England. But I'm not looking at that now.
"Even if England asked me to play for their Under-21s now, my mind is made up. I fully believe it's the right decision."
Phillips has been forced out of the Scottish party for Wednesday's friendly in Slovenia due to injury but he is expected to be included in Craig Levein's squad when Scotland face the United States in Florida on May 26.
"Unfortunately I've had to pull out of the Slovenia game because of a slight niggle in my hamstring but if I'm selected for the next squad, hopefully I can get my first cap then," he said.
Source: AFP
Source: AFP