It's built into the phone. Network support is determined by the protocols and frequencies that are supported by the phone's hardware. Network protocols and frequencies can vary significantly by region, country and carrier. Support is also limited by the protocols and bands in use at the time of manufacture. That's compounded by some carriers selling phones that are crippled to work with only their own networks. The situation is improving, especially in high end phones, as more phones are built to work with almost all networks in their intended region. Some phones are called universal but they sometimes won't work with carriers or in countries that use unusually configured networks. When buying unlocked phones, be very careful to buy phones made for the correct region. A phone made for Europe or Asia may not works well or at all in North America.