I got it in a "from basement" condition. Unfortunately it doesn't start to any resident OS, but logic seems to work. It looks it has been repaired few times, with changing telephone plug and handset cord. Later PSU capacitors went bad and have been replaced, but these replaced capacitors seem to be used few more years. It was manufactured ca. 1986, most parts have a timestamp from 1984.

The computer can turn CRT on by logic and off using a relay located on PSU board. The internal power supply unit gives 5V, 12V and ca. 24V. There is also some high voltage for CRT module.

Description in German magazine (in German).

Disassembly:

First, check for one screw in the bottom, right behind keyboard. It may be present or not.
Then, use screwdriver to unlock front panel by putting it in two holes on the upper side of the computer. Pull the front panel towards you, it should open leaving CRT and keyboard.
You can remove keyboard by sliding it and pulling, don't break ribbon cable.
To remove cover, take it with two hands in bottom near front and pull away from the computer and upwards - it will unlock and open to the rear.
It's time to remove boards. Disconnect topmost speaker, then unlock spring rod and slide the boards off the chassis.
Removing CRT/power supply compartment is more difficult, You need to unlock two plastic bolts on the very rear on sides, then pull a whole CRT compartment towards keyboard and make it open on the front part. When it'll be in quite large angle it will be possible to remove it. Mainboard should be visible then.

If you want to remove display board only, you may need to remove high-voltage cable from CRT too. Or it will pop out by itself.

Starting

About starting, it will start and await for telephone line blinking the leftmost LED. Power it on by picking up the handset or pressing one of 3 keys with LEDs.
"Applications" can be selected with icon butons. The LED buttons have the following functions:
- First one is telephone operation. It allows to run most applications and use unit as telephone
- Second one is Bildschirmtext mode. It needs to connect to BTX-central so it will usually just wait for it... and that's all.
- Third one is modification mode - it allows to e.g. edit text in notepad, schedule or phonebook list.
Independently the leftmost function key "Loschen" works all time to delete stuff e.g. clear notepad or remove phonebook entries.

Square key near left Shift works as lternate character or (Square+Macro) displays temrinal macro list to use in BTX mode.

The last function key with "Ende" sign turns the terminal off. CRT will be blanked and if the handset is hung up the machine will turn CRT off after few seconds.

Opening the lock
The lock used in Siemens Bitel is a very simple lock. It can be picked with simple tools. You need:
1. A small needle-like flat screwdriver
2. A flat-blade screwdriver with blade about 3mm wide.
Facing the terminal's CRT, if the lock is in zero position (the hole is horizontal), it can be rotated one step (45 degrees) to the left or one to the right.
To open this lock without key, insert a needle screwdirver first and by sliding it against smaller edges determine in which one the pins are installed. Now insert the larger screwdriver to the other end, with the smaller one fully inserted.
Apply small torque using large screwdriver and carefully start to remove the smaller one while pressing pins in the hole. Do it few times. Insert in the middle of the hole, remove in the side of the hole pressing pins. If you're lucky, 3 or 4 passes will be enough to make pins "lock" in opening position. You will be able to rotate the lock full 45 degrees and you'll hear a distinct click of contacts.

The picture below shows proper alignment of tools:

CRT configuration

See Multitel Fe Ap 90-1.1