SIP trunk planning: numbers, capacity and a clean cutover

Inventory numbers and their purpose

List main numbers, direct numbers and any lines used for alarms, fax machines, lifts or payment terminals. Do not assume that a device designed for an analogue line will behave correctly over a general VoIP service. Keep each number linked to its owner, current carrier account and destination. Separate numbers to port from lines that need specialist review.

Check compatibility before purchasing

Confirm whether the carrier uses registration or IP authentication, which codecs it accepts and how it sends inbound numbers. Ask about outbound caller ID, geographic restrictions, channel limits and number-porting requirements. Match these details to the PBX documentation. A successful inbound call does not prove that outbound calls, transfers or touch-tone input will work.

Size for the busiest period

Count active calls during a representative peak, including inbound and outbound traffic. Consider campaign days or seasonal enquiries separately. Leave an agreed amount of headroom rather than buying unlimited capacity by default. Also review the internet connection and firewall with the installer: signalling and media have different requirements, and bandwidth alone does not establish call quality.

Rehearse the switch

Use a temporary number to test inbound routing, outbound identity, voicemail and transfers. Test touch-tone navigation through an external service and calls to a mobile network. Schedule porting only after the new system passes those checks. Keep a contact at the old and new carriers and confirm the escalation path for a delayed or incomplete port.

Write down the failure route

Ask whether the carrier can divert the main number when the PBX is unreachable and how that diversion is activated. Test the procedure with an approved alternate destination. In a Telfron deployment, validate the chosen trunk and routing rules in the actual environment rather than assuming every SIP service has identical behaviour.

Worked example: moving a main number

An illustrative office wants to move its published reception number while retaining direct numbers for three departments. The project owner first confirms that the number records match the carrier account and that the required porting documentation is available. The new trunk is tested on a temporary number before any port request is scheduled. Reception signs off the greeting, queue and overflow route.

On cutover day, testers call from more than one external network and verify the departmental numbers separately. They make outbound calls and check the displayed business identity. A carrier representative remains available for escalation. The team keeps a log of test times and results so that a fault affecting one number is not mistaken for a failure of the entire PBX.

Diagnose signalling and audio separately

A phone can ring while the conversation has no audio. Treat call setup and media delivery as separate checks. Capture the time, calling number, called number, affected extension and whether the fault occurs in one or both directions. Ask the installer to review the signalling and media paths using the approved diagnostic tools.

Avoid changing several firewall and codec settings at once. Compare a known working endpoint with the affected one, repeat a controlled call and document the result. A busy trunk, incorrect inbound number matching and a one-way audio problem require different investigations. Keeping these observations separate makes the carrier discussion much more productive.

Keep the number inventory after migration

The inventory becomes an operating document. Update it when a department moves, a new number is purchased or a temporary campaign ends. Include the destination, business owner and carrier account reference without placing passwords in the shared sheet. Review numbers that no longer receive legitimate traffic before cancelling them.

Also retain the tested diversion procedure and the contacts authorized to request a carrier change. A future outage may occur when the original installer is unavailable. Staff should be able to find the approved alternate route and understand which numbers it covers without reconstructing the migration project from old emails.

A trunk acceptance sheet