It is interesting, and even important for some, to have multiple phones ring on one incoming call. You might want your home phone, office phone, mobile phone, including those of your family or workers, to ring at the same time on an incoming call. To do that, you need a PBX configured for that, which is quite expensive, both as a service and in terms of equipment. There are fortunately some services our there that offer phone numbers that allow you to achieve this.
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Taking over from late GrandCentral, Google Voice has revolutionized the ‘one number to ring them all’ idea. Google Voice service is free and offers a free phone number that rings multiple phones, along with a package of many free features including voice-to-text transcription of calls, call recording, conferencing etc. Read more on how Google Voice works
3Jam offers what Google Voice does, but with a plus, and that plus costs something. Unlike Google Voice, 3Jam is open to people anywhere in the world. Also, you can have as many phone numbers as you wish. 3Jam also has more features and even an API for development. See this comparison between Google Voice and 3Jam
Phonebooth is a serious alternative to Google Voice, in that it is also free. It gives you a number in your area and allows you to receive 200 minutes of calls. It also offers voice-to-text transcription, an auto attendant and a click-to-call widget. You can upgrade to paid Phonebooth OnDemand for outgoing calls. The Phonebooth service has a solid VoIP background behind it and therefore offers very competitive calling rates, comparable to other VoIP players on the market.