Simply put, there is no such thing as a truly secure phonecall.
Any "easy" solution coming out of or running through the USA needs to be "insecure" thanks to CALEA - Communications Assistance for Law Enforcement Act - but even if this were not an issue, the endpoints can still be bugged and systems hacked.
You may be able to get a fair part of the way there by setting up your own infrastructure (ie something which runs over a VPN and/or ZRTP) - Maybe look at Silent Circle for an ?easy? partial solution to your woes.
Truth. The thing most people forget is that the NSA and similar organizations use brute-force decryption as a last resort. As the FBI has demonstrated very publicly several times over the last year that hacking the endpoint is the preferred method of intercept (after CALEA) because it's so much easier.
Windows Phone, iOS, and Android (yes, including Black Phone) all have so much surface area and so many insecure third-party dependencies that it's all but impossible to lock them down. So if someone really wants in, you're going to get hacked no matter which phone or app you're using.