When businesses deploy SIP trunks, the intention is to replace, or even displace, physical trunks from your incumbent telecommunication. Cost savings are a key driver for doing this, and the extent to which physical trunks are dropped will determine how much more bandwidth you'll need to carry this traffic. Keep in mind that legacy trunks usually only support voice, whereas SIP trunks carry all types of traffic.
If you are only using SIP trunking for voice, your current levels of broadband may be sufficient. However, if you plan to ramp up usage of other modes, especially video, you will almost certainly need to increase bandwidth. Even in terms of video, however, this additional cost will be less than maintaining your current complement of legacy trunks and having to get even more legacy trunks to support your growth.
For more information, check out SearchUnfiedCommunications.com's primer on SIP trunking.
This was first published in February 2013