Glossary / SIP Trunking
📡 SIP Trunking is the section covering how your business phone system connects to the outside telephone network over the internet. This section contains 13 terms, from the basics of what a SIP trunk is to authentication methods and caller ID security standards.
On this page: SIP Trunk · Concurrent Call · IP Authentication · Emergency Calling · STIR/SHAKEN · ITSP · Burst Capacity · Trunk Registration · Credential Authentication · Outbound Proxy · Trunk Group · DID Trunk · Number Porting
SIP Trunk
A virtual connection between your phone system (PBX) and the telephone network, delivered over the internet using SIP. It replaces traditional phone lines (ISDN, analogue) with an internet-based alternative. A SIP trunk can carry many simultaneous calls over a single internet connection. It is the most common way for businesses to connect their PBX to the outside world today.
Concurrent Call / Simultaneous Call
The number of calls that can be active on a SIP trunk at the same time. If you have 10 concurrent call channels, 10 people in your office can be on external calls simultaneously. If an 11th person tries to call, they will get a busy signal. Cloud PBX providers often offer unlimited channels for a flat fee.
Related: Burst Capacity · Trunk Group
IP Authentication
A method of securing a SIP trunk by allowing connections only from pre-approved IP addresses. Instead of using usernames and passwords, the SIP provider whitelists your office's public IP address. Any SIP traffic from that IP is accepted; traffic from other IPs is rejected. This is simpler and often more reliable than credential-based authentication for fixed-location PBX systems.
Related: Credential Authentication · ACL
Emergency Calling (112 / E911)
The ability to reach emergency services from a VoIP phone. This is a critical consideration for SIP trunking because VoIP calls do not inherently carry location information the way traditional landlines do. In Europe, the emergency number 112 must be reachable. Providers must register the physical address of each phone so that emergency services know where to send help.
Related: Regulatory
STIR/SHAKEN
A set of technical standards designed to combat caller ID spoofing (fake caller ID). STIR (Secure Telephone Identity Revisited) and SHAKEN (Signature-based Handling of Asserted information using toKENs) work together to digitally sign calls, verifying that the caller ID has not been forged. SIP trunk providers are increasingly required to implement STIR/SHAKEN to fight robocalls and fraud.
Related: Caller ID Spoofing · CLI
ITSP (Internet Telephony Service Provider)
A company that sells SIP trunking services. They connect your PBX to the PSTN (public phone network) and provide you with phone numbers. Examples include Mixvoip in Luxembourg, as well as international providers. Choosing the right provider depends on call quality, geographic coverage, pricing, and support.
Related: SIP Trunk · Number Porting
Burst Capacity
The ability to temporarily exceed your normal number of simultaneous call channels during peak periods. Some SIP trunk providers allow bursting (e.g., normally 10 channels, but up to 15 during a busy hour) and charge extra for the additional channels. This flexibility helps businesses handle unexpected call spikes without permanently paying for extra capacity.
Related: Concurrent Call
Trunk Registration
The process by which your PBX authenticates itself with the SIP trunk provider. There are two main methods: (1) Registration-based, where the PBX sends a REGISTER message with credentials, similar to how a phone registers. (2) IP-based authentication, where the provider allows connections only from your specific IP address, with no REGISTER needed. IP-based is more common for business SIP trunks.
Related: IP Authentication · Credential Authentication · REGISTER
Credential Authentication
A method of securing a SIP trunk using a username and password (digest authentication). The PBX sends a REGISTER or INVITE, the provider challenges it with a 401/407 response, and the PBX re-sends with credentials. This method works from any IP address, making it suitable for mobile or cloud-based PBX deployments.
Related: IP Authentication · Digest Authentication
Outbound Proxy
A SIP server that handles all outgoing call requests from your PBX before forwarding them to the SIP trunk provider. Some providers require you to configure an outbound proxy address in your PBX settings. The proxy may perform authentication, apply routing rules, or add billing information to the call.
Related: SIP Proxy
Trunk Group
A collection of SIP trunks that are managed together as a single logical unit. If one trunk in the group is busy or fails, the system automatically routes the call to another trunk in the group. Trunk groups provide redundancy and load balancing for higher reliability.
Related: Failover · SIP Trunk
DID Trunk / DID Range
A SIP trunk specifically provisioned with a set of DID (Direct Inward Dialling) numbers. The provider assigns a range of phone numbers (e.g., +352 2634 0100 to +352 2634 0199) that all arrive over the same trunk. Your PBX then routes each number to the appropriate extension.
Related: DID / DDI · Inbound Route
Number Porting (in SIP Context)
The process of transferring your existing phone numbers from a traditional carrier or another SIP provider to your new SIP trunk provider. The new provider submits a porting request to the current carrier. During porting, there is usually a brief cutover period. Numbers can be ported in days or weeks depending on the country.
Related: Telephone Number Portability · LNP
Related Sections
🔗 SIP Protocol — The signalling protocol that powers SIP trunking
📞 Core Concepts — PSTN, ISDN, DID, and other foundational terms
🔒 Security — TLS, SRTP, and VoIP fraud prevention
💰 Billing and Numbering — Pricing models, number formats, and porting fees
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