Choose a single number for one main route, campaign or small team. Choose a DDI block when several users, departments or services need distinct public numbers that route through the same SIP trunk or phone system. Plan the full range before ordering or porting.
What is a DDI?
DDI means Direct Dial-In or Direct Inward Dialling. DID is the equivalent term widely used in international documentation. A DDI lets an external caller reach a defined user, queue, IVR or application without first calling the main switchboard route.
A block is a consecutive range managed together. The provider delivers the called number to the phone system, and the PBX maps it to the correct destination.
When one number is enough
A single number suits a sole trader, small team, campaign, published support line or simple mobile divert. The call can still reach a queue or multiple users through a hosted phone system.
One number is easier to advertise and maintain, but it may provide less direct routing and less granular reporting than separate DDIs.
- One main sales or support route.
- Simple mobile or landline divert.
- Small hosted-phone setup.
- Campaign or tracking line.
- Temporary service or project.
When a DDI block is better
A block suits organisations that want direct numbers for people, departments, sites or applications. It can also reserve a consistent range for growth and make number administration easier.
Do not allocate every number immediately without documenting ownership. Keep a register of assigned, spare, published and retired DDIs.
- Direct numbers for users or departments.
- Separate sales, support and accounts routes.
- Multi-site or multi-brand services.
- Contact-centre queues and IVRs.
- Future growth within a consecutive range.
How DDIs are used in 3CX and SIP systems
3CX requires the DID numbers to be added to the trunk in the format received from the provider. Each number can then be assigned to a user, ring group, queue, IVR or other call-handling destination.
Exact number formatting and source identification are important. A mismatch can send calls to the default trunk route or cause rejection.
Treat the range as one migration project
Before porting, identify the main billing number, every DDI, any gaps in the range and services tied to the old account. Partial ports can be more complex than moving the complete range.
Build a mapping document from old number to new destination and test representative numbers before the migration date.
- Complete list of numbers and current destinations.
- Main billing or pilot number.
- Numbers advertised publicly.
- Unused numbers that should remain reserved.
- Temporary test numbers and fallback routes.
- Porting evidence and provider account details.
UKDDI single numbers and consecutive DDI blocks
Choose individual numbers online or contact UKDDI about consecutive number blocks and technical delivery.
Ask about a DDI block