that call to the home office only requests a charge into the account balance. If the system then sends the balance adjustment over the air, there is no way for a hacker to spoof that transmitted packet into the encrypted over the air stream.
Also, there are algorithms used to ensure validity of the sender. If the system rotates the validity checking keys via the broadcast signal, your spoofing box will not have the matching home office keys.
There are many ways to secure a system like this because of the very low probablility that one could hack the broadcast stream.