Each SIP phone requires approximately 90kbps of bandwidth to be available in both directions at the same time. This is called full duplex.
Many Internet providers provide Asymmetrical service in which the download speed is higher than the upload speed and must be taken into consideration. For example you may have a DSL connection that is 5 Mbps download, but only 768 Kbps upload. This requires you to use the upload speed to calculate bandwidth requirements.
In addition you may have a computer, laptops, tablets or cellular devices using the same Internet connection. This must be taken into consideration as well. If the computers are simply looking at webpages with some test an graphics, it will have little impact on internet bandwidth, but if used for looking at media driven websites like YouTube or file downloading this can have a dramatic effect on bandwidth and voice quality. In environments where this type of web traffic may be required, a bandwidth optimization solution will be required to provide traffic priority. Strategic Voice Solutions can help you with product selection if applicable.