Depending on the phone you have, placement of the Tap is critical:

iPhones: the NFC reader is at the top 1/4th of the phone, so your best placement options are near the middle or bottom.

Androids: the NFC reader is near the middle, so the bottom or top will work

You'll get a shipping notification whenever your new Tap is coming. You can also google your phone model to find a diagram of where the NFC reader is located.