Comment # 6 on bug 914661 from Martin Pluskal

(In reply to a b from comment #5)
> I did
> rpm -U mgetty-1.1.36-84.1.x86_64.rpm
> * systemctl start mgetty@modem.service  -- never returns
> 
> $ systemctl enable mgetty@modem.service
> ln -s '/usr/lib/systemd/system/mgetty@.service'
> '/etc/systemd/system/getty.target.wants/mgetty@modem.service'
> 
> 
> but no mgetty to find with ps
> 
> note;
>  i have tested /dev/modem by using minicom, it works

Ah I see, /dev/modem is symlink to /dev/ttyX, but this will not work with unit
due to other reasons (feel free to look at unit file) - you should use
systemctl start mgetty@ttyX.service.

Even if service fails to start, systemctl start mgetty@X.service should (and
will) finish after some time (several minutes).

After that you can see what happened via "systemctl status mgetty@X.service".