These are some of the multiple features which can produce heat in a smartphone, these include amongst others; network conditions, data usage (more if 3G/4G); any apps using audio video & data - gaming, bluetooth, GPS and of course charging.
It is not unexpected that when a smartphone is used and configured for the first time that a number of these features will be in use at the same time therefore causing the smartphone to feel warm. Generally after purchase a new owner will test all components; utilizing Audio / Video features to explore the extent to which these can be used, account configuration such as Gmail with a restoration of contacts which provokes a higher than normal level of data, WLAN / WWAN usage and not to mention the installation of multiple applications. Another noticeable effect is a reduction in the expected battery life; all of the features require power.
After examining customer useage logs it was found that the three main causes for a smartphone feeling warmer than usual were;
Frequent camera usage which also involves the cpu which runs algorithms for image enhancement.
Playing games which will put the smartphone's cpu to it's higest frequency limit for extended periods.
Using 3G/4G networs for data transfer; busy RF traffic will produce heat.
All of the above can individually cause the temperature of a smartphone to potentially rise up to 10+° C above the current ambient temperature, so if when playing an online game a phone call were to be received the smartphone could feel considerably warmer than expected especially when placed against naked skin such as an ear. A combination of the above can accelerate the time needed for a rise in operating temperature.
Taking the above points into consideration it is likely that a customer will experience a rise in smartphone operating temperature when the smartphone is new;
During the first days after intial purchase, (or factory reset after an upgrade), a customer will utilize the audio and video functions of the smartphone to full extent whilst exploring the different features.
Data usage will be high during the time that Google and other accounts are being configured and sychronized on the smartphone.
Various applications will be installed and functionality tested alongside voice calls and web browsing.
It is not possible to examine each individual usage secnario which could arise; after all each person will use their smartphone and it's features in a different manner. However, hopefully the above explains some of these scenarios and reasons why a smartphone may at times produce heat.