How to setup number with MP3 announcement?

we just installed AsteriskNOW 1.7.1 distribution, which end up with FreePBX
It is already possible to call each other.

But now I would like to have a simple number e.g. 99999
which is playing an MP3 when calling that number.

Does anyone have a simple description how to do that in detail ?

If possible without installing additional modules.