The biggest problem with mobile phone privacy (assuming you seek it) is that no matter how trustworthy and privacy respecting the operating system and its software is, the baseband modem is usually at the behest of the network provider and also generally has full access to the main CPU and memory. There is one project I am aware of that is looking to address this problem - the Neo900 [1].
The platform should hopefully be 100 % trustworthy (from an "it's free software so I can inspect it" point of view), as long as you do not choose to use a non-free graphics driver.