I have never heard of this and don't know what this could be. To dial an Italian cell phone number from the US, you dial the same as any other Italian number: - 011 for long distance - 39 country code - area code and number
Telephones all over the world have varying numbers of digits, so a # signals to the switch that you have finished dialing. It is not required, but it makes the call go through faster. A sufficiently long pause by the person dialing has the same effect.
If you have unnaturally impatient friends, or inexperienced friends who do not realize that establishing an international connection takes longer than calls within the US, then they might hang up prematurely.