Megaphones can be purchased at level 1 with Prepaid NX and level 70 with Credit NX.
There are two kinds:
Channel Megaphone - allows everyone in the channel chat to view your message.
Server Megaphone - allows every player to see this message in the Server Megaphone chat above the general chats.
Note: once one character on the account has reached level 70, all characters on that account can purchase megaphones regardless of level. However, your character must be level 40 to use.