If the number you're searching for is longer than the minimum character limit, then it should just work. For example, doing a fulltext search here, looking for 1234

SQL Query
SELECT POST_ID FROM ubbt_POSTS WHERE MATCH (POST_BODY) against ('1234' in boolean mode)

Comes up with several results, including:

https://www.ubbcentral.com/forums/ubbthreads.php/topics/151140#Post151140
https://www.ubbcentral.com/forums/ubbthreads.php/topics/150457#Post150457