If I want to have my phone back this afternoon, I will have to bring it to a repair center this morning.

If I want to have my phone back within this afternoon, I must go to fix it now. As an American, I would say it this way: I need (I'd like) to have my phone back this afternoon, so I need to go get it fixed now.

If I want to have my phone back by this afternoon, I have to go to fix it now.