Is a military service member required to call a civilian Sir?

No, but they are required to conduct themselves in a manner that does not bring discredit to the military. When dealing with government officials and employees, they should use Sir, the individuals may very well outrank them. The use of Sir is easy and there are few that would find it objectionable.
No, but they are required to conduct themselves in a manner that does not bring discredit to the military. When dealing with government officials and employees, they should use Sir, the individuals may very well outrank them. The use of Sir is easy and there are few that would find it objectionable.