There is no init script shipped with modem manager.
Created attachment 458744 [details]
modemmanager init script
The attached works for me.
3 years and counting. I'm still using the attached exactly as is ... and it's working well for me.
I get that this is aimed primarily at desktop environments, eg, gnome, but we've got this deployed even in certain remote server locations where init scripts are really useful, as evident by the fact that a systemd file is included.