From Robert Poe's article:
VoIP comes in lots of flavors, but the kind that delivers all your calls over your broadband Internet connection has the worst reputation these days. Providers collapse without warning, and there's no way to guarantee call quality. Still, such services have significant attractions for small businesses, including cost and features. If you decide to go this route, you don't have to accept all the bad along with the good. You can do a lot to help yourself, and you can ask your provider to help you help yourself. Here are some things you can do.
1. Make sure you have the bandwidth you need. One of your first self-help steps should be to make sure that you have the right amount of bandwidth. As the most successful small-business VoIP provider, with some 8,000 customers, Packet8 service provider 8x8 Inc. provides a reasonable basis for comparison. Its bandwidth rule of thumb is that 25 to 30 percent of the workers at a typical company are on the phone at any one time. Give yourself a cushion and assume it'll be 40 percent. 8x8 uses D.729 compression technology to squeeze a voice call into 25 Kbps of bandwidth. That means for a 10-employee business, you'll need 100 Kbps to carry calls alone, on top of all your other data needs. If your provider uses different technology, find out what it is and adjust your estimates accordingly.
2. Make sure you have the right kind of bandwidth. The "A" in "ADSL" stands for "asynchronous." That means that the speeds are different in different directions. That may be OK for home users who download more files than they upload. But if you do more back-and-forth business than Web surfing on your office network, it might sometimes be harder for people to hear you than for you to hear them. 8x8. recommends at least SDSL (synchronous DSL) and perhaps fractional T1 connections.
You can get here a good FAQ on SIP and VOIP.
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