On a Domain Controller, the server operator accounts must be added to the Server Operators group. On a member server or a stand-alone server, they must be added to the local Administrators group. There is no way to allow anyone not in such powerful groups to issue AT commands. Scheduled tasks are run in the security context of the Schedule service which often is allowed to execute as SYSTEM. Adding users to such powerful groups is NOT recommended. Third-party products are better approach if they do not also run as operating system powerful account.