Allophones of /b ɡ k/ can be heard as [β ɣ ʔ]. [t͡ɕ] is a distinct sound in Mentawai, but not given phoneme status by Syamsir et al. because of the lack of minimal contrasts.[3]
The semivowels /w j/ only appear in final position.
Arifin, Syamsir; Malano, Nasroel; Kasim, Yuslina (1992).
Fonologi Bahasa Mentawai [Mentawai Language Phonology] (PDF) (in Indonesian). Jakarta: Pusat Pembinaan dan Pengembangan Bahasa, Departemen Pendidikan dan Kebudayaa.