Siemens S1/S3

Here's a trick to play Tetris, also known as Klotz on a Siemens S1.
Turn the phone on. Enter PIN. Before the telephone has logged in to the your GSM net do the following steps.
1: Choose F 9 (function settings)
2: Choose send own number and push the right button for choose.
3: Push 9
USE 2 as < and 8 as > , rotate the klotz with 5 and drop with 4.
[I have just heard from one of the programmers of the phone that Tetris was in the phone during development, but it was removed before it was produced. Apparently someone has the patent on games in mobile phones and Siemens didn't want to pay the licensing fees. It was either in Menu F-9-9 or it's not there at all.]

Siemens S3 Comfort

MONITOR MODE

The Siemens S3 Com series have a Monitor mode that can be activated directly from the standard user keypad. In order to activate this mode, you must enter the special access code currently programmed into the phone.
TO ACTIVATE MONITOR MODE:
Turn the phone on.
Enter the PIN access code on press OK. (Only if is enabled).
Press Menu. (Left top button).
Press 9. (Phone Options).
Press 8. (Phone Status) (May be button 7 in some phones).
You may see your IMEI Number
Left Display Button (Top left MENU key).
7684666
Hang-Up Button. (Red Handset key).

Note: Monitor should appear now at the bottom of the Network Options menu.
MONITOR MODE

Pressing the right Display Button when in monitor mode will toggle into 1 and 2 modes

Siemens S4/S4 Power

In phonebook under "Own Phone No." enter +12022243121 (which is the number to the White House) and you'll get a greeting from the people who programmed the phone