Picture a company with ten landlines. For every new line or office move, it has to submit a request, wait for wiring and buy new equipment. At busy times, customers hear a busy signal because the number of physical lines is limited. A SIP trunk connects the phone system to the public telephone network over the data network instead of copper wires.
SIP Trunks in Plain Terms
SIP is a protocol used to set up, manage and end phone calls over an IP network. A SIP trunk is a logical connection between your phone system (IP-PBX) and an internet telephony service provider (ITSP). The provider receives calls from the public telephone network and delivers them to your phone system over the internet or a dedicated link, and carries outbound calls the same way.
With a SIP trunk, you deal with two concepts:
- Channel: the number of simultaneous calls the trunk supports. Ten channels means ten simultaneous calls, regardless of how many numbers you have.
- Number (DID): the numbers customers call. You can have several numbers on one trunk and route each to a different department or menu.
Benefits of a SIP Trunk
- Fast scalability: adding channels or getting a new number is usually done by changing the service, not by running wires and buying equipment.
- No gateways or analog cards: the phone system talks directly to the provider, removing a hardware layer that could fail.
- Location independence: if the office moves or the phone system sits in a data center, the trunk moves with it. Whether you can keep your numbers depends on the provider’s policy.
- Flexible routing: if the office goes down, calls can be routed to another branch or to managers’ mobile phones.
Technical Prerequisites
A SIP trunk is only as good as the link it runs over. Check the following before migrating:
- Bandwidth: each call uses a set amount of bandwidth in both directions, depending on the codec. The G.711 codec offers better quality but uses much more bandwidth than compressed codecs such as G.729. Calculate capacity based on the maximum number of simultaneous calls and the upload traffic of other systems.
- Link stability: latency and packet loss matter more than nominal speed. A dedicated link or a connection from inside a data center, if available, is the more reliable option.
- Authentication type: some providers set up the trunk with a username and password (Registration), and others based on your static IP address. The second option requires a static public IP.
- Firewall and NAT: open SIP ports and the RTP voice port range only to the provider’s addresses.
Never leave your phone system’s SIP port open to the entire internet. Restrict access to the provider’s published IP addresses only.
Questions to Ask Your Provider
- Which codecs does the trunk support, and does sending keypad tones (DTMF) work properly for the auto attendant?
- Is the caller number (Caller ID) passed correctly on inbound and outbound calls?
- What happens to inbound calls if your phone system is unreachable? Can they be forwarded automatically to another number?
- Can a spending cap or credit limit be set to prevent abuse?
Common Mistakes
- Removing the old lines completely before a stable trial period.
- Calculating bandwidth from download speed, when upload is usually the bottleneck.
- Leaving SIP ALG enabled on the router, which causes one-way audio and dropped calls.
- Having no backup path; when the internet goes down, the entire company phone system goes down with it.
Checklist
- The number of channels is based on the actual peak of simultaneous calls.
- Upload bandwidth and QoS for voice have been checked.
- SIP ports are open only to the provider.
- Caller ID, menu keys and call transfer have been tested.
- You have an alternative path for internet outages, such as an analog line or forwarding to mobile phones.
On a well-prepared network with secure settings, a SIP trunk makes your company phone system more flexible and more reliable. ITPlus724 engineers can assess your network for a SIP trunk and set it up on your phone system.