http://dictionary.reference.com/search?q=pound%20sign

http://dictionary.reference.com/search?q=number%20sign

According to the infallible dictionary.com, # is both a pound and an number sign. Anyone is said otherwise is wrong.

EDIT:

http://dictionary.reference.com/search?q=hash%20mark
http://dictionary.reference.com/search?q=hash

Both of those entries make no mention of # as a "hash." So anyone who suggested that is also wrong.