How to delete information from old iPhone to new iPhone?
how to factory set iphone 15 without losing my phone number
[Re-Titled by Moderator]
Sorry, this is confusing since you are asking about 2 different things.
1) how to delete information from old iphone to new iphone.
You would only delete information from the old phone if you have transferred data to a new phone. Follow the instructions in this Apple Support document: What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
2) how to factory set iphone 15 without losing my phone number
If you have transferred data from the old phone to the new iPhone 15, factory resetting the iPhone 15 would erase all of the data that you have just transferred to the new phone. Why would you want to do this?