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.