Digital mobile phones vastly reduce the cost of mobile phones by enabling many more phones to utilize the same number of towers.
After the successful launch of analog mobile phones, the need for a digital standard quickly became apparent. Digital phones are smaller, use less power, and require fewer towers.
There are two mobile digital standards. The most common, by far, is the Global System for Mobile Communications, or GSM. Additionally, Code Division Multiple Access, or CDMA, is a competing standard used in the US and Iraq.
Basically, an enormous consortium of mainly European academics and telephone companies worked together to develop GSM. In contrast, US-based Qualcomm developed and patented CDMA.
Initially, CDMA was faster, offering better call quality. However, over time, GSM caught up. GSM has significantly fewer patent restrictions, due to cross-licensing agreements than CDMA.
Markedly during this period, Finish company Nokia became the dominant seller of digital mobile phones.
Eventually, as technology evolved, GSM overtook CDMA. Subsequently, the GSM standard, with the ubiquitous SIM card, won out.