Let K be a number field with ring of integers R. Given a modulus m for K and a group Γ of residues modulo m, we consider the semi-direct product R Rm,Γ obtained by restricting the multiplicative part of the full ax + b-semigroup over R to those algebraic integers whose residue modulo m lies in Γ, and we study the left regular C*-algebra of this semigroup. We give two presentations of this C*-algebra and realize it as a full corner in a crossed product C*-algebra. We also establish a faithfulness criterion for representations in terms of projections associated with ideal classes in a quotient of the ray class group modulo m, and we explicitly describe the primitive ideals using relations only involving the range projections of the generating isometries; this leads to an explicit description of the boundary quotient. Our results generalize and strengthen those of Cuntz, Deninger, and Laca and of Echterhoff and Laca for the C*-algebra of the full ax + b-semigroup. We conclude by showing that our construction is functorial in the appropriate sense; in particular, we prove that the left regular C*-algebra of R Rm,Γ embeds canonically into the left regular C*-algebra of the full ax + b-semigroup. Our methods rely heavily on Li's theory of semigroup C*-algebras.