When you roam, you are connected to one of the mobile networks in your country of residence.
For billing purposes, operators in some countries sometimes replace certain destination numbers with generic numbers.
Most of the time, these are calls to service numbers, hotlines or value-added service numbers etc. (Orange France, for example, systematically replaces all short numbers with the generic number +333000 on the bill).
It is also possible that the operator may simply not have registered the destination number and, for this reason, indicates a generic number. Sometimes it is possible to find the actual destination number on a search site in the country concerned (e.g. Google.it, .fr, .es, etc.).
In principle, this is therefore not a billing error but a display problem on the invoice.