What prefix must one dial to call the us from the uae?

First of all, you dial a fax number on a fax machine exactly the same way you would dial the same telephone number on a landline telephone at the same location.

The telephone country code for the United Arab Emirates is +971, and Dubai's area code 04 becomes +971 4 in international format. Dialed from the United States, that would be 011-971-4 plus the local number in Dubai (7 more digits). If you place a pound sign (#) after the number, it will signal the telephone switch that you have finished dialing the full number; otherwise, the switch will wait several seconds to see if you dial any more digits, since international telephone numbers can have variable lengths.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)