VoIP Phone Line for Business | How It Works & Costs

What Is a VoIP Phone Line?

A VoIP phone line is the internet-based equivalent of a traditional telephone line. Instead of a physical copper wire connecting your business to the telephone exchange, a VoIP line is a virtual connection maintained over your broadband. Calls are converted to digital data, transmitted over the internet, and reassembled at the destination — all within milliseconds.

For businesses, a VoIP phone line is typically part of a hosted phone system (cloud PBX) rather than a standalone service. The line is represented as a SIP trunk or hosted extension within the system, and each user or call path consumes one line when a call is active.

How a VoIP Phone Line Differs from a Traditional Line

Physical Infrastructure

A traditional PSTN or ISDN line requires a physical copper connection from the telephone exchange to your premises. Capacity is fixed — if you have 10 ISDN channels, you can have 10 simultaneous calls. Adding capacity means ordering additional lines and waiting for installation.

A VoIP phone line is a software entity. Capacity is defined by your internet bandwidth and your VoIP plan. Adding more lines is typically an online configuration change with no physical work required.

Cost Structure

Traditional lines carry a fixed monthly rental regardless of call volume. VoIP is priced per user (extension) rather than per line, and call charges for UK numbers are typically included in the monthly fee. This makes VoIP structurally cheaper for most business scenarios.

Geographic Flexibility

A traditional line is physically tied to a location. A VoIP line works wherever there is an internet connection. Your business number — whether a London 020, a Birmingham 0121, or a national 03 number — rings on the same extension regardless of where the user is working.

Types of VoIP Phone Lines

Hosted Extension

A hosted extension is a user-based VoIP line in a cloud phone system. Each user has an extension number and accesses calls through an IP phone, a desktop softphone, or a mobile app. This is the most common form of VoIP line for business users.

SIP Trunk

A SIP trunk connects a business's on-premise PBX (private branch exchange) to the carrier network over the internet. It replaces ISDN30 lines and allows the existing PBX to make and receive calls via IP. SIP trunks are typically priced per channel (concurrent call), from around £4–£8 per channel per month.

Cloud Direct Routing

Used with Microsoft Teams Phone, direct routing connects a Teams environment to a carrier's SIP infrastructure, enabling Teams to make and receive calls to standard phone numbers. This is a specialised VoIP connection for Teams-centric organisations.

What Does a VoIP Phone Line Cost?

VoIP line costs vary by product type:

• Hosted extension (per user): £8–£18 per user per month, including calls
• SIP trunk (per channel): £4–£8 per channel per month, plus call charges
• Geographic phone number: £1–£5 per month per number (often included in plans)
• Non-geographic numbers (0800, 03): £3–£10 per month per number

For most SMEs, a hosted extension plan provides better value than SIP trunking, since it includes calls and all features in one per-user fee without requiring an on-premise PBX.

Getting a VoIP Phone Number

When you set up a VoIP phone line, you can choose a new number from any UK area code (geographic or non-geographic), or port your existing BT or other carrier numbers. Number porting allows you to keep all your existing contact details and published numbers while moving to VoIP infrastructure.

Providers such as Gamma, 3CX, and RingCentral maintain large pools of available UK numbers across all area codes. AMVIA can help businesses select the right numbers and manage the porting process from existing carriers.