English, unlike a language such as Russian or the other Slavic languages, does not have what is called “assimilative voicing,” which is the change of unvoiced consnants to their voiced counterparts immediately preceding voiced consonants. Thus, whereas in Russian, voiceless obstruents (= “true” consonants) like /s, t, p, k/,  etc., change to voiced /z, d, b, g/, etc. before a  voiced obstruent, English obstruents remain unchanged, hence in a word like disdain, the /s/ before /d/ remains voiceless.

However, as Y-H-B was reminded when listening to the British World Service the other day, some speakers of English do pronounce orthographic s as [z] before the consonant /l/, which is voiced as are all sonorants. In discussing the battery-driven automobile called Tesla (named after the Serbian-American scientist Nikola Tesla [1856-1943]), several speakers pronounced the word [tɛzlə] with a [z] instead of the correct [s] before the [l]. This pronunciation is in conformity with occasional items such as quisling, tousled, measly, etc., where the normal English outcome (cf. sly, misled, etc.) does not counteance assimilative voicing. It is irregular all the same.

These data are evidence of the fact that English consonants are only phonetically voiced or voiceless, their phonological characterization being rather what is called “tense vs. lax”––unlike a language such as Russian––as regards the category of protensity.