While in an online session, go into your contacts list on your phone, scroll to Merryweather and select the Backup helicopter option. You must be rank 20 to unlock this option on MW. If another player has activated it already in a session, you can't call for a backup helicopter yourself and must wait till it expires from said player. You could leave the session for a new one and try there instead. A very easy achievement to get but requires the simple grind of playing the game and reaching rank 20.