As you maybe know this model supports AT commands and can be used for gsm module. I already have one but don't know how to connect it to Arduino. If you have ideas - please share 
Do you have a serial cable for the C55? then you can connect it to the Arduino Serial port (pin 0,1) and send the AT commands...
No i don't have serial cable. As far I know this cable contains some electronics to work.
I found these schemes about serial cable:
http://forum.allsiemens.com/files/cable_c55_gprs_universal.jpg
Too complicated for me :).
Here is your pin diagram http://pinouts.ru/CellularPhones-P-W/siemens_c55_pinout.shtml
For what I am seeing you have to use pins 3 and 4. Im currently controlling 2 phones with AT commands so I may suggest you to use NewSoftSerial lib for the phone and Serial for debugging on your PC.
PS. Dont forget the GND pin. So you need 3 pins: Tx for send, Rx for read and GND for Ground.
The diagram that you posted is for direct connection to PC. If you connect the phone directly to Arduino you dont need MAX232 IC.
Thank you for this info.
Can I just connect them, or I need some resistors in between?
The mobile phone works on 3.7 volts, but if 3.3 volts are enough can I use arduino power pin 3V3 to power the phone? I know that all li-on batteries have 3 pins. Do I need to connect the third pin?
Thank you in advance.
I am not sure if it will work. Connect it directly Arduino pin <-> phone pin and test. I am using two different Sony Ericsson models and both work. If it wont work you can use something like this http://www.sparkfun.com/products/8745
PS: Sorry I misunderstood you. You want to charge the phone? I am not sure how much current the phone will drain. The Arduino pins are limited and you could fry it.
I want to send commands and to charge it in the same time.
I feel its not a good idea to charge the phone with arduino...
BTW when I turn off my phone during the night(still connected to arduino rx,tx and gnd) on the next day the phone battery is half charged. I am looking for power supply for both - the phone and the board to charge them with only one cable.
This is what I am talking about - to create a battery free GSM module.
Look what I found for my c55:
I believe that I can just use USB cable to charge the battery. And if the battery is always charged it will keep the GSM always alive. I don't need to care about the 3th pin of the battery because this is from the GSM side, not from the charged side. I am almost sure that many GSM models can be charged with 5v power from the USB. Maybe this is not the best way, but its so simple :).
Well, my phone charger's output is 450mA. I am not sure if the phone itself can drain so much but I dont wanna try otherwise it would be easy to charge the phone trough any arduino pin if the phone battery get low (AT+CBC command).
I bought a separate atmega328 micro with burned Arduino bootloader which Im gonna deploy onto separate PCB. Here is the power schematic which I created but Im not sure if this is suitable to power both the phone and all other components:
This is my voltage regulator L7805CV: ST Microelectronics 2143 datasheet pdf and if I understand the datasheet it supports Output current 1.5A.
I will use 9V wallwart adapter plugged to the power jack. The atmega328 IC will power PIR sensor,buzzer,3 LED's and 2 pins for Serial communication to the phone.
It might be good to put a thermistor before the "atmega328 power" to protect it in case of overdrain.
Im beginner in electronics so I will apretiate if someone give us some help.
To check the battery before charge it is a really good idea. You can force the phone to send sms alert for low battery, and to charge it.
If you need to use arduino without a PC you can charge it from the wall with adapter, but if you need your device to be easily movable and don't want to connect it from the wall you can use adapter like this:
http://www.dealextreme.com/p/aa-battery-usb-emergency-backup-power-and-charger-pack-15048
Most of the rechargable AA batteries serve something like 1.2v-1.3v and 1.3 * 4 = 5.2v - close to the normal USB power. You can use a transistor to close and open a connection from power source to the gsm if you need to charge it.
It's great that I can use an old GSM as time keeping device for arduino.
I am so lame in electronics and try to do thinks as simple as they can be :).