I have a land line number. I am going to X out some of the numbers so that stranger will not be calling me, but rest assured my number is something like 869XXX
However I like in county Galway so the area code there is 091. So my number is 091 869XXX.
Now Ireland is a country in a phone network, and Ireland needs to be identified by its own code. The code for Ireland is 353. So our number now becomes 353 91 869XXX, you knock off the 0 of the 091. Now to be more precise, our number is actually 0035391869XXX, the 00 is often not written in and is just replaced with a + sign, so you might see our number, if it were written on a website that needed to be able to be called from anywhere in the world it would be +35391869XXX.
Now take that a tiny bit further... some day people will colonise the moon. They definately will some day. And Mars... they definately will. So will Earth now have an area code? And the moon and Mars? And if so, will our moon be a part of the Earths area code? We might say, the Earth is the 3rd planet from the Sun, so Earths new area code is 03, so now my new number would be 0335391869XXX, or +335391869XXX.
But Mars would be 04... so if i had a business on Mars, and I wanted people to call me my number could be +4 XXX XXX XXXX XXXX
But what then of Moons? Should they have their own planetary area codes? Or should our moon be lumped in with our planet Earth? We have 1 moon so maybe our moon could be area code 01... so maybe if I were on the moon, lets say with my current number, it might be 0301 (see my planet, my moon) followed by my number... so it would be +40135391869XXX.
But then it get kinda complcated... ok, we have a nice clean planet. We have 1 moon. Mars has 2 moons.. Phobos and Deimos. So if I were living on Deimos my number would be 040235391869XXX... obvious! But what if I were living on an asteroid on Saturn? Also, we are 1 sun in a galaxy of billions! And there are billions of galaxies!
Call my brother who lives in the delta quaderant! Now thats gonna be a phone number... but I hope we have skype.. otherwise the bill is going to astronomical!