IP Office SysMonitor with only the filters "Targeting" and "ARS" from the Call tab should give a quick answer.
Also: Perhaps there is an old, no longer used, User extension in the system that happens to have a matching mobility number stored.
And there was a new User created with same mobility number: the older User will get matched before the new. - Has happened some of our customers before.
That would also be clear when checking SysMonitor.