How do I get my cell phone back from a detective that took it without a warrant?

Call and ask him ?

Why did he take it, when did he take it, was it evidence in some case ?

Go to the Police Station and ask. If that fails, get an Attorney.

Why was your cell phone taken and how was it taken without your permission?

Was the cell phone taken from you while you were in the commission of a crime? If yes, then it is obviously being held as evidence in your case.