Pretty poorly thought out movie. There's no point in handing your cellphones over when you've already gone to the victims house -> safehouse WITH that phone (your data and GPS location are constantly tracked, creating a timeline/history of where you are and where you've been). Thus collecting everyone's cellphone after the fact just doesn't make sense. Logs would clearly show you went to the victim's house, your drive route, and show the safehouse after. Furthermore you don't even need the physical device you just need to know the name of the person and the cellular provider.
If you're gonna go on a coordinated kidnap job requiring cellular communication, you'd buy a burner phone with cash. All these people would have burner phones. One of the women literally had a picture of her and her family on her mobile's wallpaper like lawl it was clearly her personal phone with all her contacts and identity laid bare & we're supposed to believe these people are professional criminals? They may as well have brought their wallets and driver's licenses with them on a child kidnapping job... Not to overly dissect the movie but if you're writing a story about professional criminals conducting a kidnapping at least make it believable. Sort of infantilized the rest of the movie for me lol. People all seemed like retarded dweebs and so does the writer TBH.