My unit is in good condition and has a key in lock. It powers on and can be operated in limited (by lack of BTX service) range.

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.

Opening the lock without key

See Siemens Bitel T3210

Starting, key operation

Let's start with a key. Look from the above of the terminal.
"-" - When the key is in horizontal position, it allows all operations of the device.
"\" - Position with right side (usually key has a ring there) towards handset - locks keyboard entirely.
"/" - Opposite position  (right side to the rear of the terminal) - locks only modifying of configuration.

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.

CRT configuration

As these machines had CRT operating for a long time, it is normal that it quickly went out of operation parameters. Picture may be smaller, larger, too bright or misaligned. The drawing below shows where on CRT board (looking at the metal part on the left side of terminal) different regulation devices are placed. If you tune picture height, remember that usually people see the machine from a bit above, so having the front cover makes the picture on the top edge of CRT a bit lower.

Links:
http://btxmuseum.de/hardware/geraet.asp?id=23 - In BTX Museum