[Ask] is there any Guide to setup SMS / Whatsapp / Line incident push notification.

Hi John,

 When you add an email address to a Notification Policy, you can choose a method, either email or SMS from a pulldown menu

 For SMS, for example, if the recipient is using AT&T Wireless, then use the recipient's 10-digit wireless phone number, followed by @txt.att.net
For example, 5551234567@txt.att.net. The format for the email-to-text email address is different for the various carriers.

https://www.att.com/esupport/article.html#!/wireless/KM1061254?gsi=5re8bo

For your reference, please find the Policy based notification:
https://help.fortinet.com/fsiem/6-6-3/Online-Help/HTML5_Help/FortiSIEM_Operations_Incidents_and_Cases.htm?Highlight=sms#Notifyin

Hope this information helps.