I made VOIP, Hosted PBX Solutions, Phone System Guide/Review in US and Canada businesses. We used, Industry leader Linksys, Avaya, Panasonic, Cisco, Polycom are VOIP products. It is use to connect the telephone (cellphone/home-phone/business phone) to the internet phone system.
All IP or Internet Phone like Polycom phones are connected directly to the internet. To function via VOIP phone system. Now, if you want to use your old phones you can still do that. By connecting your phone to an Adapter or Linksys Adapter to old phones connected to the high-speed internet.
In most cases VOIP consumes 100kbps internet bandwidth. So make sure you got more bandwidth (high speed internet) before you use VOIP system. To ensure good call quality. Most horror VOIP stories in the past made like poor voice quality or choppy lines.
Is caused because of slow internet connection. That was 10 years ago VOIP now is the mainstream and everybody are switching on it. Even major carier like Bell, Rogers, Telus or Verizon. You wont jeopardize your business for a minute poor business lines right? So i have a solution for you. On a month to month basis and not tied you up in a 1 year or 3 year contract.

