Permissions for phone extensions

I'm trying to find info on permissions to dial a certain extension. I don't seem to find any info on the knowledge base.  We've created an InformaCast device under Phone System Management to access InformaCast's DialCast feature that allows to send an All Call to all (or certain) speakers.  We only want school office admins to be able to dial that particular phone extension.  I can't seem to find info on how to create those permissions.