Hi,
a few years ago my doorbell died 3 times in about 5 months. The problem was that the button of the bell did not open sometimes and the coils of the bell did melt.
After a few weeks i kinda liked not having a doorbell so i did not repair it since then. And now my Girlfriend starts bugging me that i need a doorbell

So i think i make it a bit more complicated and funny

The plan:
If someone rings the doorbell it calls me on my cellphone.
If i answer the call i want to speak with the person at my door.
If i dismiss the call or it runs into a timeout it should tell the person that nobody is home ("the person you are ringing is currently not availaible"

)
I can open the door by sending a code by sms to my doorbell.
As my girlfriend needs a ring too i need to call out on 2 lines simultaneously.
Status: nothing yet
MCU: Atmel Atmega8 (should do the job)
Phone: Siemens SL55 (got a lib to control the phone)
Problems:
Yet i got only 3 Problems.
First is that i do not know how to call on 2 Lines simultaneously.
Second is that i do not know exactly if i can connect the 2 doorbell signalcables directly to the microphone input of the phone (communication AND ringing signal uses the same 2 Cables).
Third and last until now is that i did never play back a soundfile with a MCU (i think i need to save the file on a SDCARD as tts would be such an overkill ^^)
Timeplan:
I try to start the project in August and finish it also in August. I got some other stuff going on but this project does not seem to be THAT heavy.
Please give me some feedback on how i can solve my problems and so on