The main requirement is a broadband Internet connection 
such as DSL or cable. Any other equipment such as a 
telephone adapter or microphone usually comes with the VoIP 
service provider.

Using your DialUp Internet Connection also you can use VoIP.

Sam.

generally following things are required for voip

1)Broadband connection
2)voip phone
3)nexton softswitches
4)router
5)audiocodec
6)astric server