It is not usual for consonants to change from voiced to voiceless in English. However, there are a few cases where this happens. The phrases have to and has to are often pronounced hæf tu and hæs tu with voiceless consonants rather than voiced ones at the end of the first word. A similar phrase is used to, which is often pronounced juːs tu. The phrase of course is sometimes əf kɔːs rather than əv kɔːs.