Upgrade to new iPhone

I went on the mobile site to upgrade from my current iPhone 12 Pro 256GB to the iPhone 15 Pro 256GB. I wanted to trade in my iPhone 12 Pro which brought down the monthly price to right around $17 per month. The issue is that a pop up appears that tells me I don’t have an existing line with Xfinity, when I’ve been a customer for several years. So when I choose the add a new line option instead of upgrade an existing line, but choose the keep my own phone number option, I get the errors that my number is in use. So I get told I don’t have an active line but then also get told I can’t keep my number because I have an active line. Please help me make sense of what’s going on and tell me how I can upgrade to a new phone by trading in current device. Thank you.