OsmocomBB is an Free Software / Open Source GSM Baseband software implementation.
It intends to completely replace the need for a proprietary GSM baseband software, such as
- drivers for the GSM analog and digital baseband (integrated and external) peripherals
- the GSM phone-side protocol stack, from layer 1 up to layer 3
In short: By using OsmocomBB on a compatible phone, you are able to make and receive phone calls, send and receive SMS, etc. based on Free Software only.