Plan the placement of components inside the telephone. I had no problem fitting the SIM800 mini board with antenna on the right, and the rest of the stuff on the left side. Using perf board, you can create a nice and compact PCB to fit in there, maybe even screw in place with a bracket.

Program your Arduino Pro Mini. You need to download and install the SIM900 library first. Then you edit the project Arduino code to set these two things:

1: The kind of rotary dial you have. In Sweden where I live, the dial starts with 0 and ends with 9. This means that one pulse sent is a zero, and ten pulses equals number nine. If this is the case with your phone, you don't have to change anything. Otherwise, you edit the code where it says..

digit = pulses - 1; // one pulse is zero, ten pulses is nine (swedish system)
// for systems where ten pulses is zero, use code below instead:
// digit = pulses % 10;

//digit = pulses - 1; // one pulse is zero, ten pulses is nine (swedish system)
// for systems where ten pulses is zero, use code below instead:
digit = pulses % 10;

Hook up your Pro Mini using the FTDI programmer and upload the sketch.

Solder stuff together (or use pins and dupont connectors where suitable) according to the schematic. Using a socket for the Pro Mini is a good idea. Makes it easy to replace it or remove it for re-programming.

Disconnect the receiver wires from the screw terminals in the phone. The mic and reciever positive wires are to be connected to the MICP and SPKP connections of the GSM board. If you have a GSM board with 3.5 mm jacks for mic and speaker, the MICP and SPKP are TIP on the TRS (Tip/Ring/Sleeve). The ground wire of the receiver goes to GND.

Disconnect the wires coming from the wall plug and remove the cord from the phone. Connect the ring and tip connection wires from the SLIC board to the ring and tip terminals in the telephone.

Charge your battery. Put a SIM card (with PIN code disabled) into the GSM module. Set the power switch to "on" position. If there are lit LEDs and stuff looks ok, try calling the number of the SIM card from another phone. The bell should ring. If you pick up the receiver, there should be sound in both directions. If you hang up, the call should be disconnected. That was the first test. Second test is to pick up the receiver (there won't be a dial tone), dial the number of your other phone and wait... Now... if these things won't work, you are in for some fun troubleshooting (to begin with, just send me a message asking for assistance). If everything works.. congratulations! Enjoy! :)