VoIP phones can receive their in a few different ways. Power Over Ethernet (aka: POE) is one of the most common ways in corporate environments. Additionally, an injector or a traditional power adapter can be used if POE is not available.
This video demonstrates, at a basic level, how each three of the powering methods works. This is a good primer for anyone who wants to get a general understanding of what POWER OVER ETHERNET is.
Popular VoIP phones all use these methods: Polycom, Cisco, Yeahlink, AudioCodes. Moreover, the concepts go beyond just VoIP phones. Other network devices such as Wifi Access Points, IP cameras use POE or Injectors as well.
POE is particularly helpful in that it eliminates the need to have an external power supply at the location of the end point.