windows phone app =--- Accessible calculator for vissually disabled

I would suggest voice interface.

You see, general-purpose voice recognition is hard to implement, because of limited quality of the available engine, but this is still possible, maybe with certain percentage of mistakes, especially if the grammar is small — this is not dictation. If the calculator is simple, the required grammar is so small, the the recognition would show pretty good fidelity, if the user's pronunciation is at least clear, even with considerable accent. I experimented well enough with such interfaces, so I can tell it for sure.

And voice synthesis is not a problem at all.

You need to use the assembly "System.Voice.dll". It comes with redistributable .NET package, so it is in the GAC, so you will add the reference from the tab ".NET" of the "Add Reference" dialog. Please see System.Speech:
http://msdn.microsoft.com/en-us/library/system.speech.recognition.aspx[^],
http://msdn.microsoft.com/en-us/library/system.speech.synthesis.aspx[^].

You can freely download required language pack from here: http://windows.microsoft.com/en-US/windows/downloads/languages[^].

The visually impaired one will only need to activate your application and then use the voice mode only. How about that?

—SA