===== Mitel/Aastra/DeTeWe 6x0 System menu =====
  -  prex "esc" 2 times
  -  prex the >>> key
  -  type: ""***76# "

===== Fritz!Fon MT-F/C5 Secret Menus =====
Prerequisite: Phone turned off
=== Menu 4: DEVICE TEST ===
  - Hold key 4
  - Turn phone on still holding key 4
  - Hold Key 4 until menu appears

This menu holds tests for Keyboard, Display and Audio, Reset to Factory Defaults, DECT Monitor, Headset (MT-F only), Sensors (C5 only)

=== Menu 5: TESTMODE ===
  - Hold key 5
  - Turn phone on still holding key 5
  - Hold Key 5 until menu appears

This menu holds a radio test mode. No idea what this is good for.

=== Menu 6: Development-Tests ===
  - Hold key 6
  - Turn phone on still holding key 6
  - Hold Key 6 until menu appears

This menu holds tests for ambient lighting, accumulator, infrared (if present), display, colors, RAM and proximity sensor

===== Gigaset service menu =====
   - Hold 1 + 4 + 7 while powering on the phone
   - Enter the key sequence 76200