A number is not 3CX-compatible merely because it can be delivered using SIP. Confirm the provider method, authentication, source identification, DID format, inbound routes, outbound caller ID, codecs, firewall rules, simultaneous-call limits and support responsibility before migration.
What compatibility should mean
Compatibility means the provider and 3CX can reliably identify the trunk, match the called number, exchange supported signalling and media, and route calls as intended. A generic SIP registration alone is not a complete compatibility test.
3CX recommends supported providers because templates and test requirements reduce configuration risk. A generic trunk can work, but the administrator carries more responsibility for parameters and troubleshooting.
Confirm authentication and source identification
A trunk may use registration credentials or IP-based authentication. Registration-based services require the correct registrar, authentication ID and password. IP-based services require stable public addressing and provider allow-listing.
For inbound calls, 3CX must associate the SIP INVITE with the correct trunk. Source-identification settings and the provider's SIP headers matter, especially when more than one trunk from the same provider exists.
Match the DID number format exactly
3CX routes an inbound call by comparing the called number received in the SIP message with the DIDs configured on the trunk. Formatting differences are a common cause of calls falling to a catch-all route or being rejected.
Agree whether the provider sends E.164 format, national format or another defined presentation. Configure every DDI consistently and test the main number plus representative numbers from any block.
- Main trunk number.
- All individual DDI numbers.
- Called-number field used by the provider.
- Leading plus sign and country code treatment.
- Caller-ID format for outbound calls.
Create inbound and outbound routing deliberately
Assign DIDs to users, ring groups, queues, IVRs or other destinations. Configure opening hours, holidays, no-answer routes and a catch-all destination that does not expose the system to unexpected calls.
Outbound rules should define which users or departments can call which destinations and which caller ID is presented. Restrict high-risk international and premium destinations unless there is a genuine requirement.
Test signalling, audio and failure scenarios
Passing one inbound call is not enough. Test inbound and outbound calls, two-way audio, DTMF, transfers, hold, voicemail, caller ID and the maximum simultaneous calls. Confirm what happens when the internet connection, PBX or trunk is unavailable.
Run the 3CX firewall checker where applicable and verify SIP and RTP paths. Record the working configuration and provider escalation details.
- Successful registration or IP authentication.
- Inbound call to main number and DDIs.
- Outbound national and mobile calls.
- Correct caller ID and return-call behaviour.
- Two-way audio and DTMF.
- Concurrent-call limit and busy handling.
- Backup routing and incident contacts.
UKDDI 3CX-compatible number and SIP delivery
Order a UK business number with SIP trunk routing details collected for technical provisioning and validation.
Choose a number for 3CX