So the transfer of the number that is currently on. My iPhone 13 ifoing to cost me if I transfer same number to another decode k already have ?

So if I transfer the number on my currently used phone that I currently use to the other phone that I use only over Wi-Fi will it charge me to phone bills because the number can stay the same but I could use the number on both phones so I’m confused if that’s an extra charge or if it’s free to have the same number on two phones since they’re both T-Mobile and purchase there?