Normal zeta functions of the Heisenberg groups over number rings I - the unramified case

Let $K$ be a number field with ring of integers ${\mathcal{O}}_K$. We compute explicitly the local factors of the normal zeta functions of the Heisenberg groups $H({\mathcal{O}}_K)$ that are indexed by rational primes which are unramified in $K$. We show that these local zeta functions satisfy functional equations upon the inversion of the prime.