I have written about Silent Circle a couple of times, first at http://goo.gl/ntE24c and then later at http://goo.gl/PnKg30. The company provides an encrypted communications service with easy-to-use tools available to all who appreciate a little control over who has access to their conversations. It encrypts cell phone calls, text messages, and video chats with other Silent Circle members. It works on iOS (iPhone, iPad, and iPod Touch) as well as on Android cell phones and tablet computers. Yes, it is possible to make private phone calls from an iPad, a device that is not a cell phone.
Silent Circle must be highly secure as it has now been approved secure enough to be used by US government personnel for lower-level encryption although not for the more sensitive military or diplomatic uses.
I find it amusing that the U.S. government endorses a civilian-developed encryption service at a time when the same government is fighting against the use of encryption by civilians. What is good for the goose apparently is too good for the gander.
The app, called Silent Phone, is now certified as complying with the FIPS 140-2 validation, a certification required for encryption used in any software-based product deployed on US government networks. It is listed in the government’s list of approved Validated FIPS 140-1 and FIPS 140-2 Cryptographic Modules at http://csrc.nist.gov/groups/STM/cmvp/documents/140-1/1401val2015.htm. Certification is administered by the Cryptographic Module Validation Program (CMVP), operated jointly by the U.S. NIST Computer Security Division and their Canadian counterpart, Communications Security Establishment Canada (CSEC).
You can read more in Silent Circle Achieves FIPS 140-2 Encryption Validation To Meet U.S. Federal Requirements For Silent Phone on the Silent Circle web site at https://goo.gl/q71kYm.