SIP Trunks Explained: Replacing PRI Lines

For decades, businesses with many phone lines used PRI circuits: dedicated digital lines carrying a fixed number of simultaneous calls. SIP trunks deliver the same capability over IP, with more flexibility and usually lower costs.

What a SIP trunk is

A SIP trunk is a virtual connection between your phone system (IP PBX) and the voice provider's network, carried over a data connection. Each trunk supports a set number of concurrent calls, called channels.

PRI in brief

A PRI line is a physical circuit providing a fixed number of voice channels. Adding capacity means installing more circuits, which takes time and money.

Advantages of SIP trunks

• Scalability: add channels through a configuration change rather than new hardware.
• Cost: no dedicated physical circuits; calling plans are often cheaper.
• Flexibility: numbers can be routed to any site or failed over to another location.
• Fewer devices: no PRI cards or gateways needed for modern IP PBXs.

Requirements

SIP trunks need a reliable data connection with enough bandwidth and quality-of-service prioritisation for voice. Many businesses use a dedicated voice VLAN or a separate managed link.

Sizing channels

Estimate how many calls happen at the same time during your busiest hour, not total staff. A 50-person office might only need 10–15 channels.

Security considerations

Protect PBXs from toll fraud with strong passwords, restricted international dialling and firewalls limiting SIP access to the provider.

Migration approach

Port existing numbers, run SIP and PRI in parallel briefly and switch over once call quality is confirmed.

The bottom line

SIP trunks modernise business voice: easier to scale, more resilient and often cheaper than PRI lines. UbiFi Business Landline supports SIP trunking for IP PBXs.

Learn more about Business Landline & Voice from UbiFi, or talk to our team.