Moon sighting is extreme responsibility. one have to testify in front of committee that he/she saw the moon under the oath, not everyone is eligible, only noble people can do (By noble I mean those who got good reputation when it comes to telling truth, dealing with people).
Its not like you call and say.. "Hey... I think I saw the moon... and they'll be like.. WOW.. this is pretty sweet (add some emoji's)" and announce Ramadan from next day