In the Standard Model (SM) baryon number – lepton number (B-L) is perfectly conserved. Therefore, the observation of the transition from a neutron to an antineutron reveals the existence of physics beyond the SM. I will start with the Lorentz invariant B-L violating operators of lowest mass dimension and discuss the “wrong” CPT problem associated with their transformation properties under CPT. Then I will argue how the special transformation properties of Majorana fermion under the discrete symmetries CPT, CP, and C can help solve this problem and lead to the discovery of discrete-symmetry phase constraints on the fermion fields in B-L violating theories. I will discuss how the incorporation of these phase constraints leads to new ways of discovering neutron-antineutron transitions.