/usr/share/dbus-1/system-services/org.freedesktop.ModemManager.service is in modemmanager 0.5.2.0-0ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | [D-BUS Service]
Name=org.freedesktop.ModemManager
Exec=/usr/sbin/modem-manager
User=root
|