Tmobile math

I was going to trade in my 12 Pro for a 15 (iPhones both) 

their ad show $830 credit and pay $0

But they claim that my phone is valued at $155

When I thought it was worth $830 ( because that’s what they are advertising )

So now they claim they are paying $155 and they subtract it from the $830 

Which now they claim that $675 Divided by 24 is the new number 

and because the new phone is $830 they divide $155 by 24 

so what is happening is they pay $155 and take it back over a 24month period 

So in reality they are getting the $155 back