ModemManager is a system daemon which controls WWAN (2G/3G/4G/5G) devices and connections.
Alongside NetworkManager, ModemManager is the default mobile broadband management system in most standard GNU/Linux distributions (Debian, Fedora, Ubuntu, Arch Linux…), and is also available in custom systems built with e.g. buildroot, yocto/openembedded or ptxdist.
ModemManager may also be used in routers running openwrt (integrated with netifd as a new protocol handler), or to manage both voice and data in mobile phones running postmarketos (PINE64 PinePhone, BQ Aquaris X5, OnePlus 6…).
libqmi, libmbim and libqrtr-glib are additional libraries that are developed under the umbrella of the ModemManager project, but which can also be used independently with their own command line tools.
Read the documentation
This website provides multiple resources that will let you understand how the ModemManager daemon works and how it can be used either from other applications or from the command line.
Contribute changes or report bugs
The project sources and issue trackers are maintained in the freedesktop.org gitlab instance.