A recent survey by Nokia Siemens Networks finds that the "The number of people likely to switch mobile operators is now 39% globally and has jumped by more than 20% in one year .. the study underscores the importance of both Internet quality, which has become a deciding factor in the choice of operators in mature markets, and voice quality, which is key in other markets".
NSN surveys "8,700 interviews of customers in mature markets (Denmark, South Korea, Japan, UK and US), in-transition markets (Brazil, Colombia and Russia) and emerging markets (India) during the second half of 2012".
"..The impact of smart devices on usage was most pronounced in mature markets, where two out of three people are now active users of advanced data services. For these customers, mobile broadband quality is a key deciding factor in their choice of operator. Voice quality remains the chief consideration in emerging and in-transition markets".
See "Mobile operators: keep your customers loyal by focusing on voice, data quality" - here.