SIP trunks are connections that link a business IP-PBX to the PSTN (Public Switched Telephone Network). Through an (ISP) Internet VoIP connection signaling goes from the business phone system to the SIP (Session Internet Protocol) provider who connects and establishes voice calls. SIP trunks are much more cost effective than traditional types of phone services and can be used for lower costs, including lower bulk minute rates and lower costs for particular regions or countries.
SIP trunk service is significantly less expensive than traditional business telephone lines. Sometimes as much as 60 to 75% lower.
The number of SIP trunks needed by a business will depend on the type of business (call activity) and how the provider labels an active call connection, but would be one channel or trunk for each active call. Some providers offer a single trunk that can carry multiple channels, each of which would carry one call.
Configuration of SIP trunks can be done by tech savvy people and providers often offer guidance.
A small office with three people would need one trunk per person if all three would likely be on the phone at the same time. As a typical office gets larger the ratio of trunks per person would decrease to about sixty percent. Call centers and high call volume businesses would need a high ratio of trunks to active employees.