Tethering is the process of connecting a non-mobile device like a desk or laptop to a mobile device or vice versa for the purpose of accessing the internet or internet signal. Many new mobile phones can be used to tether their internet signal for your computer similar to the way a router works. For example, if you have a laptop and are not near your usual WiFi signal you can use your mobile phone’s data connection to gain access to the web.
The phones with VoIP capability happen to also offer tethering features. VoIP calls are made on WiFi signals so it makes complete sense that a 3G phone like an iPhone or the new Google G1 Android phone can be used for tethering.