Two Factor Auth. via SMS

Hello. I'm working in a government that have 3000 workers. We are trying configure an SSL VPN with tunneling mode. We want sms authentication when establish VPN connection. We don't want add 3000 users with phone number to fortigate. We want use Active Directory phone number attiribute for each user. How can I do that? or is there any other suggestion for get rid of adding thousands of user to fortigate?