Your broadband connection is quite important. The minimum required for VoIP is ideally 256Kbps download, though today's connections are generally much faster than that. The bigger the pipes at home the better the VoIP service as it reduces the chance of voice traffic having to compete with other (larger) data files such as MP3 or images. Typically, a VoIP call will consume about 20Kbps of bandwidth.
Speed is not the only thing that matters. The actual broadband service is also critically important. Ultimately, a VoIP service is totally dependent on a reliable broadband service provider to help deliver voice traffic. If a user's ADSL/cable/wireless broadband service provider has continual dropouts or outages, it will obviously affect the VoIP service. You cannot surf the Internet and open e-mails when there are outages. Nor can you make VoIP calls.
It is worth noting that if users choose to do away with their telephone provider, they are still required to pay the telephone line rental for the ADSL-enabled phone line. This is because ADSL is an add-on service for telephone subscribers. If users are on cable, they would be subject to the cable provider's subscription rates.