ModemManager Telephony functions
ModemManager is a GPL-2.0-or-later & LGPL-2.1-or-later FreeDesktop project stated in the year 2008 with customer USB dongles support for the desktops. It integrates with the higher-level NetworkManager network management daemon.
ModemManager is currently lacking a manual testing tool similar to oFono Phonesim. Phonesim support could probably be implemented on the ModemManager or ModemManagerQt side if needed.
For unit testing ModemManagerQt provides fakemodem backend.