Format
It has to go in the Control Source of the field on the report.
In other words, nothing is changed in the table itself, the field is only formatted for use on your report.
A problem may be the field name you are using.
"Number" is a reserved word in Access, rename the field in the table to something like "NumberID". Never use field names like "Number" or "Date", it may cause problems at some point.
Also it is good policy to base forms and reports on queries, and not pull data form the table directly.
Good Luck. Always glad to help