iPhone "no cell coverage" draining solution even if cell phone is turned off

Hello all,

i understood that iPhone searches for a connection when there is no cell coverage. it forces its energy to find a signal. ok cool thank you iPhone.

I work often in my cellar with no cell coverage. I automatically switch "cell phone connection" off and "wifi on. I thought that would be the solution but no, iPhone still tries to find a cell phone signal right?

how can I disable that or am I in the wrong?