Although there is a magic tool that can be used for long distance calling, if there is no magician available, there are terribly few people who can manufacture it and thus it is impossible to satisfy the demand.
Of course this was collected as well. Within the army magicians, those who were able to use long distance communication were assigned and distributed to each unit. It could be said that this was a security policy of the kingdom.
As a matter of fact, my teacher had taught me the long distance communication magic as well.