In this work, we implement the semi-analytical WKB method to explore the behaviour of a scalar field on a traversable wormhole space–time with a Casimir-like complexity reported in Eur. Phys. J.C 82, 420 (2022). We estimate the error in the computation of the quasi-normal frequencies of the scalar field at each order in the WKB and show that the order with the best accuracy is not unique. We compute the value of the quasinormal frequencies for the smallest estimated error. As an aside, we find that the imaginary part of the quasinormal modes frequencies approach to zero for the fundamental mode mimicking the so-called quasi-resonance observed for massive scalar fields in the Reissner–Nordström background.