The number is likely being modified by the selected Outbound Route or Trunk.
Look at the Dial Patterns for the Outbound Route. If the number dialed matches a pattern with anything in the prefix field, the digits matching the prefix pattern will be removed, before prepending anything in the prepend field. The result is then sent to the trunk(s).
In the Trunk, if you have any Dialed Number Manipulation Rules (usually you wouldn’t), then a matching rule with a non-empty prefix field will also cause digits to be deleted.