Phonetically complex consonants are regarded as units, such as /mb/ and /gb/, series of prenasalized and co-articulated stops as in other Ubangian languages. But I consider /kw/, /gw/, on the one hand, and /by/ and /gy/, on the other, to be comprised of sequences. The sequence /Cy/ occurs only as a variant of /Ci/.
Only /kw/ and /gw/, sequences of stop and /w/, occur. As the result of contraction, the phonology of Sango of the youth especially is quite different from what it used to be and unlike the phonologies of other Ubangian languages. For example, the preposition /ti/ can occur as /tm/ with /m/ carrying the high tone of the preposition. Another example is the word kumase (with a high frequency of occurrence) from French commencer: /koma, kmase, kwase, kme/.
Source: Bouquiaux et al. 1978: 433