Most wireless service providers will allow you to purchase phones and additional services and put the charge direct on your monthly bill instead of having payment at the time of sale. While this can be convenient, in recent days there have been several situations that I am aware of where criminals have been attempting or have successfully gained access to wireless account information and used that information to purchase phones and services that are billed direct back to the account. Basically a no cash/card transaction leaving the real account user holding the tab.
In order to protect yourself, I HIGHLY RECOMMEND contacting your wireless provider and having them place a password on your account. This password must be known by the caller in order to make purchases or modifications to your account without knowing the password. It's a simple thing to do but can save you a LOT of headache.