You have assigned a German (+49 country code) fax number to one of your users and they are unable to receive faxes on this number.
There is a known issue affecting German fax numbers due to wide-scale infrastructure and telephony policy changes in Germany where current fax and voice infrastructure is transitioning from analog/physical to strictly digital/VoIP. This has resulted in faxes to/from multiple German origins failing due to their VoIP service lacking compatibility with both the g.711 and T.38 codecs.
Open a Support request mentioning the number that is unable to receive the faxes and our Support team will get back to you after the appropriate assistance from the backend team.
Support agents should escalate this issue to the Concord team and mention the possible issue related to the g.711 and T.38 codecs, since the Concord team would swap the mentioned fax number from primary T.38 to primary g.711.