One criticism levelled at such studies is that it is difficult to tell if mobile phones are promoting growth, or growth is promoting the adoption of mobile phones, as people become able to afford them. It is easy to imagine ways in which mobile phones could stimulate economic activity-- they make up for poor infrastructure by substituting for travel, allow price data to be distributed and enable traders to engage with wider markets, and so on. Furthermore, phones do this without the need for government intervention. Mobile phone networks are built by private companies, not governments or charities, and are economically self-sustaining. Mobile operators build and run them because they make a profit doing so, and fishermen, carpenters and porters are willing to pay for the service because it increases their profits. The resulting welfare gains are indicated by the profitability of both the operators and their customers. All governments have to do is to issue licenses to operators, establish a clear and transparent regulatory framework and then wait for the phones to work their economic magic.