The custom ISIS apps, which include unbreakable 256 bit AES encryption Apps which are designed to encrypt the data on a data card which can be easily removed from the burner phone and moved to a new one, making switching phone numbers at the drop of an identity dead easy, are NOT going to be found on Apple's App store, but they are found on Android third party stores.
They do NOT recommend using an iPhone or any phone that requires being registered with a standard carrier using a credit card and an ID. High end phones such as the iPhone linked to a carrier require too much traceable information to sign up with a carrier which WILL provide the authorities with all the phone calls and messages into and out of the device, as well as the emails received and sent. The phone number is attached by name. It is ANYTHING but anonymous.
Burner phones can be purchased anywhere for as low as $6 to as high as $50, depending on their capability. . . and makes it very economical to have several if necessary and damned easy to just smash the phone when convenient.
WAKE UP AND PAY ATTENTION!