Log in onto http://infamouspapertrail.com/ and open your profile. Start examining proofs you get in the fourth mission, which are: Origami Dove #3, Crime Scene Photos and Heaven's Hellfire Poster. At the end, choose Fishbein's Phone and try to get to his voicemail. Unfortunately, you need a password.
If you want to guess the numbers via the "official method", login into DUP Intranet, by clicking in your profile or inserting the address http://departmentofunifiedprotection.com/intranet. After logging in, insert the number written on the Origami Dove #3, which is 34856135421.
Click on the button to send the code. You get an Origami Dove #4. Scroll down to the very bottom of the site and look at the picture showed on the screen above. Follow dots and write down numbers to get a new FAN Number: 36544572885.
Switch again onto the main site of the DUP Intranet and insert the number you've received. This let you unlock the Security Protocol Document. After reading it you know that there is a code: there is 3 added to each number of the code.
You can return to the main profile on the http://infamouspapertrail.com/ and choose Fishbein's Phone. Look at the current code displayed by the token and add 3 to each its number.
Very important remark: The code includes the last digit of the value, what makes thing more difficult when it's 7, 8 or 9. 7+3 is 10, so you have to put 0. 8+3 is 11, so you have to put 1. 9+3 is 12, so you have to put 2.
Exemplary code and solution is shown on the screen above. When you get a confirmation of completing the mission, head for where you begin the fifth optional mission.