Apps Exchanging data between two andrid devices using Internet

[prashant_c]

Hi,
I want to develop a App in which i want to exchange data between two devices (deviceA & deviceB) . I am assuming my App in installed on both devices.
Now the main concern is i have only phone/mobile no. of my friend with whom i want to exchange data using internet only(not through Bluetooth or sms or wifi).
Scenario will be Like
1) I will enter a number and message and send it
2) If my friend has this app installed on his device will get a notification
3) He will have option of decline or accept message.
4) if he accepts it a default message for acknowledgement will be sent to my device.
So i am stuck how to communicate using only gsm number and make sure my app in installed on his device.
Thanks in advance.