If that's the case, NUMBER is a wrong data type for that column - it should be VARCHAR2.
Depends on what "starts with zero" means
I understood, the numbering should start with the number zero.
But you are right as well. If it means "each sequence of numbers should start with the character zero" then this could either be done by storing that character when using VARCHAR or by simply formatting the number with a leading zero during retrieval.
I still don't think that NUMBER is the right choice (at least, here where I live).
For example: my telephone number is 654-321. I live in Zagreb. Zagreb's city code is 1. It means that - when someone calls me (and doesn't live in Zagreb) - has to dial "1-654-321".
On the other hand, my friend lives in Split and his telephone number is 987-654. Split's city code is 021, so I call him by dialing "021-987-654".
From those two examples, it appears that (even without formatting numbers with dashes) NUMBER column would contain "1654321" (which is OK) and "21987654" (which is wrong). VARCHAR2 would contain "1654321" and "021987654" (both OK).