Does Xfinity allow existing customers to purchase phone outside of XM?

My wife’s iPhone 7 broke. I wanted to purchase a iPhone 11 with 256 gb but XM doesn’t offer that. I purchase one else where but now I’m ready XM won’t allow me to swap the phones and keep it on the same line. This is crazy am I really being forced to pick whichever phones they offer. Ohh and remember I need a phone now.