I have seen around the internet devices like this: https://www.cryptophone.de/ and software that reinforce the security on cellphones. My question is simple, since the cellphones use A5/1, A5/2, A5/3 encryption algorithms why to use more encryption? Aren't A5/1, A5/2, A5/3 secure?
The encryption algorithms are proprietary, and little research has been performed on them. But those that has been researched are considered broken; A5/1, A5/2, A5/3... all of them are either badly implemented or fundamentally flawed.
Also, telephones tended to trust the network they connect to, because base stations was expensive. Stingray IMSI catcher exploited this. With software defined radio, such attacks has gone from prohibitively expensive for an individual, to requiring a few hundred USD.
In addition, they only encrypt traffic over the air. The telecom provider will have access to the clear text of the traffic. Additional End to End encryption removes this hole; you no longer has to trust your service provider - they can't leak the clear text, as they don't have it.
End to end encryption bypasses all problems mentioned above. As long as secrecy of the transmission is your goal, you don't have to trust the network. You only need to trust the two devices you're transmitting your data between.