Telephone dial in doesn't always work

I now had the problem several times, that the dial in via Teleophone didn't work. (Germany)

The participants wrote their Meeting ID into their Telephones but simply weren't dialed into the ongoing meeting.

It seems to me, that this happens more often when a "spontanous meeting" is startet (either form chat or contacts)

Is there any difference between a plannend and a spontanous meeting regarding the telephone dial in?