Few fields are as rife with jargon and abbreviations. Here's a guide to some of the terms you will encounter:
Centrex: a PBX (see below) hosted at the local telephone exchange
DID: Direct inward dialing; lets inbound calls go directly to extensions without an operator
PBX: Private branch exchange, a telephone switchboard used exclusively by a private organization
POTS: Plain old telephone service, i.e., a standard analog line
PSTN: Public switched telephone network, the standard network
QoS: Quality of service, an Internet protocol that gives voice traffic priority over data traffic
TDM: Time division multiplexing, a label for traditional, circuit-switched telephone equipment
VoIP: Voice over Internet protocol, a standard for transmitting voice communication via the Internet
vPBX: Virtual PBX, a scaled-down service for inbound calls
For a clear explanation of VoIP mechanics, see communication.howstuffworks.com/ip-telephony.htm.
See voipproviderslist.com for a worldwide roster of VoIP hosts, including nearly 700 in the U.S.
At shopfordsl.com, you can compare prices for broadband Internet access in your area. (Note: The results are not always exhaustive.)
Billzilla.com offers comparison shopping for long-distance plans and T1 lines, among other telecom services.