DID numbers explained: Direct Inward Dialling for business phone systems

Who this guide is for

Audience: Business buyers and phone system administrators managing number inventory.

Understand how VoIP calling works — SIP, PBX, codecs, trunking, DID numbers, STIR/SHAKEN, and the protocols behind business phone systems.

Use this guide when you want the setup to be correct the first time and easy for another admin, manager, or supervisor to verify later.

What this workflow helps you accomplish

A DID (Direct Inward Dialling) number is a virtual phone number that routes directly to a specific extension, user, or IVR destination within a PBX or VoIP system without requiring a live operator to transfer the call. DIDs decouple the phone number from the physical phone line.

This workflow matters because numbers, routing, access, and reporting in CallOrbit are connected. Skipping one setup detail usually creates avoidable support work later.

• Step 1: Understand how DIDs differ from traditional lines: a traditional phone line is a physical copper pair with one fixed number. A DID number is a virtual address that routes to an IP destination. One SIP trunk can terminate hundreds of DIDs without any physical wiring changes.
• Step 2: Map DIDs to destinations in your PBX: each DID can route to an auto attendant menu, a ring group, a queue, a specific extension, a voicemail box, a conference bridge, or an external forwarding number. This is how a single business system can handle sales, support, and directory lines separately.
• Step 3: Buy DIDs for local presence: you do not need a physical office in a city to have a local phone number there. DID numbers are available in most countries and area codes. A business in New York can have a London DID (+4420...) that rings the same team without any UK infrastructure.
• Step 4: Distinguish DIDs from toll-free numbers: DIDs are local or national numbers where the called party pays for inbound termination. Toll-free numbers (800, 833, 844, 855, 866, 877, 888 in the US; 0800 in the UK) reverse the billing model so the called party pays for the call, not the caller.
• Step 5: Manage DID inventory carefully: each provisioned DID typically carries a monthly rental fee. Audit your assigned DIDs quarterly to release unused numbers, reallocate numbers that are routing to disconnected destinations, and consolidate overlapping coverage to reduce monthly costs.

Setup checklist

• Understand how DIDs differ from traditional lines: a traditional phone line is a physical copper pair with one fixed number. A DID number is a virtual address that routes to an IP destination. One SIP trunk can terminate hundreds of DIDs without any physical wiring changes.
• Map DIDs to destinations in your PBX: each DID can route to an auto attendant menu, a ring group, a queue, a specific extension, a voicemail box, a conference bridge, or an external forwarding number. This is how a single business system can handle sales, support, and directory lines separately.
• Buy DIDs for local presence: you do not need a physical office in a city to have a local phone number there. DID numbers are available in most countries and area codes. A business in New York can have a London DID (+4420...) that rings the same team without any UK infrastructure.
• Distinguish DIDs from toll-free numbers: DIDs are local or national numbers where the called party pays for inbound termination. Toll-free numbers (800, 833, 844, 855, 866, 877, 888 in the US; 0800 in the UK) reverse the billing model so the called party pays for the call, not the caller.
• Manage DID inventory carefully: each provisioned DID typically carries a monthly rental fee. Audit your assigned DIDs quarterly to release unused numbers, reallocate numbers that are routing to disconnected destinations, and consolidate overlapping coverage to reduce monthly costs.

Operational follow-up

After you complete this flow, confirm the live experience from both the agent and customer side so ownership, routing, permissions, and reporting all match what the workspace expects.

If your team is rolling this out across multiple users, queues, or phone numbers, pair this article with the broader knowledge base and the relevant routing or numbers guides to keep deployment consistent.

• What is the CallOrbit Knowledge Base for? — It is the public help hub for how CallOrbit works, covering numbers, webphone setup, SIP, extensions, routing, users, roles, and billing basics.
• Can customers read this without signing in? — Yes. The Knowledge Base now lives on a public route so customers can read setup guidance before or after they enter the portal.
• Does the portal still have its own Knowledge Base page? — No. The signed-in portal navigation no longer carries a separate Knowledge Base page, and the old portal path now redirects to this public version.
• What is VoIP and how does it work? — VoIP (Voice over Internet Protocol) converts analogue voice signals into digital packets and transmits them over IP networks. Unlike traditional PSTN phone lines that require dedicated copper wiring per line, VoIP calls use your existing internet connection, which makes them cheaper, more flexible, and easier to scale.
• What is SIP trunking? — SIP trunking is a virtual connection that replaces traditional analogue phone lines or PRI circuits. A SIP trunk carries multiple concurrent voice channels over a single IP connection to your PBX or phone system, eliminating per-line hardware costs and monthly line rental fees.
• What is the difference between hosted PBX and cloud PBX? — Hosted PBX runs on dedicated virtual infrastructure managed by a provider, while cloud PBX uses shared multi-tenant cloud infrastructure. Hosted PBX suits organisations needing custom configuration and predictable pricing. Cloud PBX is better for instant scalability and per-user monthly billing.
• What is a DID number? — A DID (Direct Inward Dialling) number is a virtual phone number that routes directly to a specific extension, IVR menu, queue, or user within a phone system without an operator. DIDs decouple the phone number from the physical phone line, so you can have hundreds of numbers routed through a single SIP trunk.
• What are G.711, Opus, and G.729 codecs used for? — These are VoIP codecs that convert voice into digital data. G.711 uses 64 Kbps for toll-grade quality and is the PSTN standard. Opus uses 6-510 Kbps and adjusts to network conditions. G.729 uses 8 Kbps for bandwidth-constrained links. The right codec depends on your available bandwidth and call quality requirements.

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