Depends. If you don't already have a firewall or some kind of secure connection get it. VoIP can be hacked. Also check your network. It shold be at the level your provider wants. We have H.323. Pretty standard. If you don't have it, upgrade.

It really depends from provider to provider.

Broadband Internet Connection is pretty standard.
Some providers will actually sell you the network to go along with your VoIP service.

If that's not really what you're looking for, you can always try for an open standards provider.

I know OnSIP is a service provider that just requires you to bring a Broadband Internet Connection and your own VoIP phones. There may other services similar to this one.

Source: http://www.onsip.com

As far as equipment goes, the type of systems that you currently use for traditional landlines are public switched telephone networks, or PSTN. These landlines have geographic origins, and are the reason you have long-distance and international charges as well as peak-calling hours. The handsets used by PSTN connections are analog phones, and work by converting sound into electrical vibrations that are transmitted across phone lines – think two cups connected by a thin wire in terms of technology. In VoIP service, voice is transmitted as data “packets,” and these packets hold digital information such as the sound and video that VoIP systems use. Your business can either look into specific VoIP phone systems that are built to process these packets of information, or you can use an analog telephone adaptor, or ATA, to convert the information to your traditional analog handsets.

Source: http://www.ResourceNation.com