Schematic for DLR-3 cable

If you connect MICIN (pin3) with 15k resistor to logic ground (pin 9), you get 618x phone to show "READY FOR DATA CALL" message and after that it shows "DATA CONNECTION ENDED". When you short pin3 to GND with 15k resistor, phone switches pin4 (what is normally SIGNAL GROUND) to be +3V source for microprocessor, what is embedded in to the cable. After this phone sends (1F/48/00/4E/00/02/01/02/xx/check sum) message to micro (serial settings are 9600Bd, 9bit, odd parity) from MBUS pin (pin6). If you don't acknowledge the phone, then your connection is canceled with "DATA CONNECTION ENDED" message. Micro needs to send to the phone "1F/00/48/7F/xx/check sum" message every time when phone sends something to the micro. After this you can communicate with phone using "AT" commands trough the FBUS (pin7 = RXDATA, pin8 = TXDATA, 19200Bd, 8bit, no parity). There is more things going on in MBUS (like data flow control codes and much more), but this can give you a good kick start. If you use 1k - 10k resistor, phone goes to the "headset" mode, with 20k - 30k phone goes to the "test mode".

61XX pins