I took a model A raspberry pi  a tft screen  and a Arduino gsm
shield  to create a "mobile phone" 
It was a fun experiment to do, and in the end it did gave me a "phone" i
Unfortunately, it became so big and heavy, that it is more something to
put on your desk as a clock. 
Some of the reasons to build a phone myself were:
-1 because you can ;-)
-2 the pi is made in large quantities and has a big community.
-3 up-to-date kernel
-4 huge collection of programs in the raspbian repository.
I hope it inspires someone to build a better model, smaller and with a
better battery life.
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