Vorstelijk(e?) Telefoon in Prins Bernhard's (Prince Bernhard) Office

For those too young to know, that white thing is a rotary telephone.

 

My dictionary says vorstelijk = regal. But since I don't know if telefoon is mascline or feminine or neuter, I don't know if I should say vorstelijk telefoon or vorstelijkE telefoon to mean regal telephone.

Done