Is this your recent experience with American wireless carriers? I have experience with phone companies and this is not how the big ones work. They did work like this years ago but it's been a while since they have. The big ones right now have a plan per line and additional charges based on add-ons and device payments. So it'll be something like $80 phone plan + $20 device payment (for 3 years) + $8 insurance. Once the payment plan is over the plan and addon's typically remain the same price. If you bring your own phone as a NEW customer the big carriers give you a discount up front or distributed through a monthly credit, but the plan is the same plan. Once the credit runs out it's the same price as everyone else's plan.
Example bills:
New customer with no promotional offers: $25 iPhone 15 (I don't know the actual price for this) $80 unlimited plan $8 insurance $113 total
New customer with trade in promotion: $25 iPhone 15 -$25 iPhone 15 $80 unlimited plan $8 insurance $88 total (I'm not a Nazi I'm just pretty sure this is the price of the cheapest insurance option)
Existing customer who buys a phone from Craigslist: $80 unlimited plan $8 insurance $88 total
New customer that brings their own Craigslist device: $-5 credit for bringing own phone (probably applied for 24-36 months) $80 unlimited plan $8 insurance $83 total
This would be nice to know, my mom asked me like a decade ago and I was unable to find anything back then.