Free phone operating system
Replicant is a fully free Android distribution supported by the FSF through its Working Together for Free Software Fund. It is the first mobile operating system (OS) to run without relying on proprietary system code.
Ways to help
Use Replicant and become part of the project's community by using its forums, contributing to its wiki, submitting any bugs you discover, and sending patches to improve Replicant. Please donate via the FSF (Replicant's fiscal sponsor) to help the project grow by supporting more devices and advancing to Replicant 11.0.
You could also help existing FSF certified GNU/Linux distributions to support smartphones. You can do this for example by contributing directly to these distributions and adding support for specific smartphones (for instance when the smartphone is already well supported by upstream projects). Or you can contribute directly to various upstream projects to support smartphone hardware or to adapt GNU/Linux applications to work well on smartphones*.
*Smartphones are very different from laptops. They typically don't have any hardware keyboard or mouse, but instead they use touchscreens. These are way less precise than a mouse pointer as they are typically used with (relatively big) fingers, and because they often have very small displays with very high resolutions.
This is just one item on the Free Software Foundation's High Priority Projects list.