Not the right solution
I considered buying this product for my missus last Xmas, because she's always forgetting her phone. But it's poorly designed - read some of the many reviews on the web, eg, those complaining being awoken at night when the phone ran out of power. Plus it needs to be charged, etc.
The right solution - for her at least - is the Bringrr (bringrr.com). This lives in the car, plugged into a power outlet, and beeps one of two tones when the car is started, depending on whether it detects the phone's BT signal or not. If the phone isn't there, you won't drive off without it. Simple. And a lot cheaper, and totally no-hassle.