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.
This project is doing amazing work, but despite all the effort, it only supports very small number of phones based on one particular baseband chip because this one happens to accept unsigned firmware. It only supports 2G (and not even completely), so 3G and 4G are completely out of the question. Don't expect to flash this on your Samsung Galaxy Whatever any time soon.
Aside from the immense technical knowledge, expertise, and dedication required to code your own baseband software, there's a huge legal barrier - it's pretty much illegal to use a baseband like this without explicit approval. In fact, the people behind the project do not use their software on carrier networks.
Despite the fact that the need for a properly open source baseband firmware is obvious to everyone, the cold and harsh truth remains that we're not even close.