FortiVoice LDAP Query (samAccountName)

I can't quite seem to get the FortiVoice LDAP Query to work correctly. We can only use the default 'Active Directory' Query: (&(objectClass=user)(telephonenumber=$u))

However, it doesn't help us much because we'd have to go in and manually change at least 250 AD user accounts to change their full number to the four digit extension (and they want the full number).

I've tried different queries like (objectClass=user)(cn=$u)), (objectClass=user)(samAccountName=$u)), and even (&(objectCategory=person)(objectClass=user)(sAMAccountName=$u))

When I test the LDAP query and type in my username, which is in the form of: lastname.firstname it says that the LDAP query test was successful.

However, when I try to login to the FortiVoice web gui using: firstname.lastname and my AD password it says: "Login failed. Please try again". 

However the telephonenumber=$u method will work correctly, as long as I type in my four digit extension and that it's not a number longer than that.

Am I using the wrong query string? Is there an issue with periods in the username? Is it just not working at all? We updated our FortiVoice firmware to the latest, so you'd think it would work.

Is there a way to see LDAP related logs from the FortiVoice to see why the LDAP query test will test successfully, but when actually logging in, it fails?