Why is the letter В sometimes pronounced as Ф?

Although Russian is generally very phonetic, letters in a word sometimes appear to be pronounced not as they are written. This is all due to a few pronunciation rules. Once you know them, you will be able to read and say anything without fear of mispronouncing the words.

One rule that we are going to discuss now is “consonant devoicing”. Technically speaking, ‘devoicing’ means that a voiced consonant ‘loses’ its voice and becomes voiceless. For instance, the voiced В [v] is pronounced like Ф [f].

However, it only happens when:

1. the voiced consonant occurs at the end of a word, e.g. часов

           you see: часов         you say: чисоф
     

2.  when different consonants come together (forming a voiced + voiceless cluster) they tend to assimilate, i.e. become similar in sound. The preceding consonant changes to accommodate the next sound. For instance, in the phrase в пять, the voiced в [v] precedes the voiceless п [p] and thus changes to a voiceless consonant. 

         you see: в пять           you say:     ф пять
                        повторите                      пафтарити


This rule applies to all other voiced consonants, not just В. Here is the complete list of all voiced consonants with their voiceless pairs:

Voiced        Voiceless         You see:            You say:

Б                 П                        хлеб                 хлеп
В                 Ф                       вчера                фчира
Г                  К                       маг                    мак
Д                 Т                       год                     гот
Ж               Ш                       муж                   муш
З                С                        узкий                уский

Also, a word like двенадцать.  Is the letter ц also voiceless since д sounds like a т? In other words, is that another pair?