[Glossary](https://telxpress.com/glossary/) / Phones # SIP trunk A **SIP trunk** is a virtual phone line delivered over the internet that connects your own phone system (an on-site or hosted PBX) to the public telephone network, replacing traditional analog or digital carrier lines. - Phones - Updated October 11, 2026 - Written by the Telxpress advisory team 01 In depth ## What it is, why it matters and *what to ask a vendor* SIP, the Session Initiation Protocol defined in RFC 3261, sets up and ends calls between systems. A SIP trunk is a carrier service that uses it: your PBX sends outbound calls to the carrier over your internet connection, and inbound calls to your numbers arrive the same way. Capacity is sold as concurrent call channels rather than physical lines. Growing businesses with a PBX they still like (Avaya, Yeastar and similar systems) use SIP trunks to cut line costs and add capacity without replacing the system. It is also a step toward the cloud: numbers move to a carrier that can later point them at a hosted platform or an AI voice agent. Ask a carrier how many concurrent calls are included and how bursts are billed, whether the trunk is registration-based or IP-authenticated, how E911 is provisioned for each location, what failover happens when your internet is down, whether your PBX is certified with them, and who configures the firewall rules. 02 Related terms ## Terms that usually come up *alongside it* - Phones ### [PBX](https://telxpress.com/glossary/pbx/) A PBX (private branch exchange) is a business phone system that switches calls between internal extensions and the outside world; it can be a box on your premises or a hosted service in the cloud. - Phones ### [VoIP](https://telxpress.com/glossary/voip/) VoIP (voice over internet protocol) carries phone calls as data over an internet or private IP network instead of traditional copper phone lines; cloud phone systems, UCaaS and SIP trunks are all built on it. - Phones ### [UCaaS](https://telxpress.com/glossary/ucaas/) UCaaS (unified communications as a service) is a cloud subscription that combines business phone calling, video meetings, team messaging, voicemail and often contact-center features in one platform managed by the provider. - Connectivity ### [Failover](https://telxpress.com/glossary/failover/) Failover is the automatic switch to a backup connection, device or route when the primary one fails, so that phones, payments and cloud applications keep working through an internet or equipment outage. 03 At Telxpress ## Where this shows up *at Telxpress* The pages and articles where this term does real work: what we advise, build and support. - Page ### [Business VoIP & Cloud Phone Systems in Irvine](https://telxpress.com/solutions/business-technology/phone-systems/) VoIP, cloud phone systems, UCaaS and SIP trunks from Yealink, Nextiva, Avaya, Yeastar and Polycom, installed, supported and ready for AI call handling. - Page ### [Business Internet, Fiber, SD-WAN & Wi-Fi](https://telxpress.com/solutions/business-technology/connectivity/) Business internet, fiber, SD-WAN, Wi-Fi and carrier services for Orange County offices, selected and managed by one local partner with 24/7 support. - Page ### [Business Networking, Firewalls & Managed Wi-Fi](https://telxpress.com/solutions/business-technology/networking/) Switching, routing, Fortinet firewalls, Ubiquiti and Cisco Meraki Wi-Fi and managed network services for offices that depend on reliable connections. 04 Sources ## Public *references* The standard or law this definition rests on, as published by its owner. Fetched and checked on the review date above. - [RFC 3261, SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261) rfc-editor.org Next step ## Want This Applied *to Your Business?* Book a free 30-minute AI readiness consultation. You'll leave with three AI opportunities specific to your business, whether you work with us or not. [Book My Free Consultation](https://telxpress.com/book/) [Take the Free AI Readiness Assessment](https://telxpress.com/ai-readiness/) [Or call 24/7: (949) 861-4500](tel:+19498614500) Last reviewed: October 11, 2026. Published by Telxpress, Irvine, CA.