upgrading my daughters Apple Watch to a new iphone

Our 11 year old currently has cellular service on her AppleWatch and has memorized that "phone number" (she currently does not have a separate iPhone). We want to get her a new iPhone, but she wants to keep that same number. Do we have to get a "separate line" and request the number be transferred? Or is it considered an upgrade to the existing line? We have AT&T and I'll be calling them later, but was hoping to get it solved before then since it's a surprise. Thank you!