Three of the larger players among VoIP services are Skype, Verizon Business Services, and Vonage. Skype offers first-rate VoIP services to small and large businesses in addition to VoIP services for private users. Verizon Business Services offers a wide array of VoIP services for businesses, at competitive prices. And Vonage is a major presence as a VoIP provider for small and medium businesses.
One of Vonage's service options, Small Business Premium Unlimited ($50 per month), includes unlimited local and long-distance calls within the United States, Canada, and Puerto Rico, along with land lines in many European countries. It also supports having your Vonage number to ring up to five existing lines; and among its other features are voicemail, Caller ID, a dedicated fax, and 500 minutes of outgoing calls. On top of that, you can take your Vonage adapter with you and be connected from anyplace that has an Internet connection, just as if you were in your office.
If you don't want to take your Vonage device with you, you can extend the plan to any mobile phone line you specify. An app for Android and Apple devices offers even more features.
Accompanying Vonage's SBPU plan is the Vonage Phone Adapter, which allows you to use existing telephones through the Vonage VoIP system. Actual Vonage phones cost from $15 to more than $100; and in my hands-on testing, the service was well worth the price of admission.
Affordable as Vonage's service is, it is by no means the cheapest one available. Local VoIP suppliers sometimes beat the prices of the big guys, so take time to shop around in your area.