Repair
The back of my phone cracked can i get it repaired no charge ?
Question marked as
No.
You should arrange a service or repair
https://support.apple.com/iphone/repair/service
Anything other than Screen or Battery may be a replacement phone and the price is in the article.
If not covered by warranty and you don't have AppleCare+, you may have to pay full replacement cost for "severe damage"