SIP trunks are, to be non-technical, phone lines over the Internet.  They come at great discounts compared to traditional telephone lines, and so many organizations save money on telephone service by using SIP trunking.

If a customer wants to use SIP trunks, please know that they can be included to many telephone systems as extra lines without abandoning their traditional lines.  This way, SIP trunks can be tested for quality and reliability before porting, or transferring an organization's main number to the SIP trunk provider.