It won't sync automatically. You'd have to set up a system that manages the process for you.
Easiest is to keep the database on a computer so that it is hosted by either FileMaker Pro or FileMaker Server. Your iPhone users then use WiFi, 3g or 4g to connect to the database as clients of the hosted database. Since the data is kept on the server, there's no sync needed--but this assumes that your iPhone users will have a good wireless connection to your database.
I've heard mention of a third party sync tool over in the FM GO forum. (See tabs at top of this screen.) I don't know if it works with FM Go 12 or not nor how well it works, but you might inquire over there and see what you can find out.
It's also possible to set up an import records operation that merges a copy of your database on the iPhone with the data on your computer. This requires scripting and, depending on how many different users might be modifying different copies of your database and also in how complex your database is, this process could be quite simple or very complex.