VoIP has quietly been gaining ground as a new communications system for about 10 years in the form of services like Skype and Vonage. It's available in most large- and mid-sized American cities because it piggybacks on T1 and DSL service lines, which are nearly universal in population centers. (Verizon alone has programs in 350 cities [source: Verizon.com].) But even though it potentially frees a business from landlines and can connect multiple, remote users into the same outgoing network, it's not actually available everywhere just yet.
When you research IP trunking, keep an eye out for SIP trunking. This is like IP trunking, but with a slight difference in technology that increases the power of the phone system and allows for more data transmissions. This increased power in an SIP trunking system is used for advanced telephony services, such as voice mail, directory assistance, and 900-number blocking -- all important business needs.
One big problem with VoIP and IP trunking is the relative lack of security. It's not yet as secure as a landline. Landlines provide direct, physical connections of voice to voice. VoIP calls are data traveling by wire and through the air. So potentially, they can be backed into or accessed by someone -- either from within your own IP trunking network, or outside of it. In the early days of widespread cell phone usage, it was common to overhear other people's phone calls.
And even where it is available, services, bandwidth capacity and voice quality vary. So, research different providers before you sign up for anything.