Comment # 6 on bug 968573 from Dieter Jurzitza

Ok you are totally right. Doing a

systemctl enable mgetty@modem.service

creates a link:

mgetty@modem.service -> /usr/lib/systemd/system/mgetty@.services

in /etc/systemd/system/getty.target.wants, so this part of the story is clear
now - user error, but how to know this? Thank you very much for that hint!

But we are not readily done. The link "/dev/modem" is created from within
/etc/init.d/boot.local because noone can anticipate that the modem - device
remains always the same.

Now, knowing where the link "modem" points to, I can do a

systemctl enable mgetty@ttyS4.service
systemctl start mgetty@ttyS4.service

and I will be done, mgetty is working for now. But, as I said before: I cannot
be sure that "ttyS4" will always be the modem device. On boot I am asking for
the right device via "hwinfo" and create the link (/etc/init.d/boot.local).
Therefore the link makes absolutely sense, however, in this case systemde
fails:

systemctl enable mgetty@modem.service
systemctl start mgetty@modem.service

fails (the link being present and pointing to /dev/ttyS4). The operation is
timing out, and I get the following messages:

Feb 28 17:48:56 djunix systemd[1]: [/usr/lib/systemd/system/dm-event.socket:10]
Unknown lvalue 'RemoveOnStop' in section 'Socket'
Feb 28 17:50:02 djunix systemd[1]: [/usr/lib/systemd/system/dm-event.socket:10]
Unknown lvalue 'RemoveOnStop' in section 'Socket'
Feb 28 17:51:40 djunix systemd[1]: Job dev-modem.device/start timed out.
Feb 28 17:51:40 djunix systemd[1]: Timed out waiting for device
dev-modem.device.
Feb 28 17:51:40 djunix systemd[1]: Dependency failed for mgetty on /dev/modem.

Could you kindly shed some light on this, too? Things are going well if I
delete the reference to 
BindsTo=dev-%i.device
from within mgetty@service, so, "ttyS4" is acceptable, a link "modem->ttyS4" is
not? How would I ensure the apropriate device to get selected?

Thanks again - and, finally, could someone provide a pointer where to get this
information from? The documentation is difficult to seek for this.