Officially, Vodafone represents the phrase "Voice Data Phone", and the history on what appears to be a number six is provided as follows:
SOURCE
"In 1997, Vodafone introduced its Speechmark logo, composed of a quotation mark in a circle, with the O's in the Vodafone logotype representing opening and closing quotation marks and suggesting conversation."
But I ask, could there be a hidden or alternative meaning? Consider the symbolism used by Vodafone and the number of "the beast" from Revelation 13 in the Bible. Typically, the beast is seen as representing a negative character, and the number 666 represents evil.