Redstone has no means of communication to the outside, so even if you created an entire computer with redstone(very unlikely) you would have no means of communication outside of minecraft.
That would be a neat experiment to see if the conjecture, "Redstone has no means of communication to the outside" is true. It would be interesting to find bugs within the redstone rendering engine that leak or emit data. I'm thinking of something similar to the Super Mario World bank switching pong game.
That would be using a security vulnerability to reprogram the game then, of course if you can do that you can do anything on the computer. Since Minecraft is sandboxed in the JVM that might prove difficult though.
Well, if you have Computer Craft (or presumably Open Computers) you can get redstone out of the game and send an SMS:
https://boomtree.com/r2p
I made that. But just this week LittleBits released a Minecraft mod called "bitCraft" that lets you use their CloudBit to simplify all these steps even further:
http://discuss.littlebits.cc/t/bitcraft-our-minecraft-mod-is...
Reverse engineer the client code such that a little bot player runs around in game flipping levers and seeing if redstone torches are on/off. Then you would truly have a computer with a little guy running around inside it doing all the thinking.
The famous (is it?) minebot already implements about 99% of the required functionality and requires no server side mods. There are of course competitors to minebot some of which may be closer or further away from this goal.