assume that you have two android devices (GSM enabled) paired via Bluetooth (and/or WiFi direct)
this application is to use one of the devices to enable voice calls over the other device.
By this way if you are using one gsm enabled android tablet and one android phone with regular size and If someone calls your tablet, you will be able to receive calls over android phone or vice verse. And also making calls by phone over tablet can be feature.
Please let me know if there exists an application doing this, or a developer that may work on that.
I can prepare more detailed requiements for the sw.