It's pretty abvious but it took me some time to figure it out.
What does number 3 on the phone have to do with the word def?

Take a look at number 3 on the phone:

-------------------
|  1  |  2  |  3  |
|     | ABC | DEF |
------+-----+------
|  1  |  2  |  3  |
| GHI | JLK | MNO |
------+-----+------
|  1  |  2  |  3  |
|PQRS | TUV |WXYZ |
-------------------

I like this phrase.
It's plain stupid and just not to geeky so anyone can understand it.