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Cusp transitivity in hyperbolic 3-manifolds

Let $M$ be a cusped finite-volume hyperbolic three-manifold with isometry group $G$. Then $G$ induces a $k$-transitive action by permutation on the cusps of $M$ for some integer $k\ge 0$. Generically $G$ is trivial and $k=0$, but $k>0$ does occur in special cases. We show examples with $k=1,2,4$. An interesting question concerns the possible number of cusps for a fixed $k$. Our main result provides an answer for $k=2$ by constructing a family of manifolds having no upper bound on the number of cusps.