FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
This article describes the free SMS credit given to FortiGate and how to check it.
Do take note that SMS is not a free service, and the free SMS credit here will only give when the condition is met.

There are several usages for SMS which includes the following:
1) Sending Fortitoken Mobile activation code to user.
2) Using SMS directly for 2FA.

By default, the FortiGate will obtain free SMS credit when Fortitoken Mobile license is registered. 

The calculation is as follows:
2* (No. of FortiToken mobile license purchased) = Total SMS credit obtained.

It is basically possible to check the SMS credit value using the following command from the CLI (represented by 'SMS max allowed' value):

# execute fortiguard-message info

If the SMS credit had met 0, it will be necessary to update the SMS license. 
Itis basically possible to update the SMS credit using the following command:

# execute fortiguard-message update <activation code>

Important Notes.
1) Free SMS credit (after purchased from FortiToken Mobile) will need to be use within one year or else it will be invalid due to expiry.
2) TAC support will not be able to provide extra SMS activation code. check with therespective local Fortinet Sales team for further enquiries.