If freedom is your goal and you're willing to make some compromises (like
running a proprietary bios) then you can use a smartphone, but you should buy
a phone that is supported by replicant.
Cyanogenmod is besides replicant afaik the best choice at the moment; at
least your system is rooted. Remove the google play store and install f-droid
instead; remove all proprietary apps and run only free ones.
Of course, using replicant is much better and i would recommend that!
Now, if you're really concerned about privacy - if you're a journalist or
political activist, for instance - then don't use any cellphone.
I think the only thing you can use under those circumstances is a thinkpad
with a free bios, a replaced wifi card which doesn't require any non-free
components and tails as an operating system.
Tails ships with proprietary firmware so you really have to watch out that
your device won't use it, i.e. that every component works with free firmware.
Still you will be vulnerable to hardware backdoors and a few pieces of
proprietary firmware on the device, such as the hard disk firmware.
Considering all that, i actually would advice against using any pc or any
technology whatsoever - if you really can't make any compromise when it comes
to privacy.
