How to detect Hidden Camera in changing rooms

In front of the changing room, take out your mobile phone and make sure it can make calls.

Then enter the changing room and try again to make a call...
If you can't make a call...
There is a hidden camera...

This is due to the interference of fiber optic during the signal transfer...

If this is true (unsure of the fiber optic mentioned), then it would help prevent us from embarrassing STARRING ROLES IN PASAR MALAM VIDEOS!

I am also wondering about the chances of being able to detect if let's say, those hidden cameras are uncommon. How many more times would you try out after a number of negative attempts? In the first place, we are unsure if this will work.
Link.