Core concepts

A short glossary of the terms you'll meet everywhere in this documentation. For the full telecom glossary see didhub.io/glossary.

DID (Direct Inward Dialing) number

A real, locally-dialable phone number — what most people call a "phone number." A DID isn't tied to a physical line at a specific address; it terminates over the internet (via SIP) to whatever endpoint you configure.

Sometimes called: virtual phone number, SIP number, VoIP number, cloud phone number. They all mean the same thing.

SIP trunk

The protocol-level connection between DIDHub and your phone system. One SIP trunk can carry signaling + audio for many DIDs.

Think of it as a pipe:

• Inbound: calls to your DIDs arrive over the trunk
• Outbound: calls placed from your system go out over the trunk
• Capacity: measured in concurrent calls / channels

A DIDHub trunk has:

• An FQDN (e.g. sip.didhub.io) and port (5060 UDP / 5061 TLS)
• Authentication: IP ACL or SIP digest
• A list of bound DIDs (the numbers that route to it)

Routing

The decision of where to send an inbound call when it arrives at a DID. Options:

• SIP trunk — forward to a SIP endpoint (most common)
• PSTN forward — forward to another phone number (replaces traditional call forwarding)
• Webhook (IVR) — fire an HTTP request and let your code decide
• Microsoft Teams Direct Routing — terminate into a Teams tenant
• Zoom Phone BYOC — terminate into a Zoom Phone tenant
• Voicemail-to-email — record the message, email the recording

STIR/SHAKEN attestation

A SIP-level signature on outbound calls that says "the originating carrier verified this call." Three levels:

• A (Full) — carrier verified the customer is authorised to use the calling number. DIDHub signs all originating PSTN traffic A.
• B (Partial) — customer verified, number not verified. Aggressively flagged as "Spam Likely."
• C (Gateway) — pass-through, no customer relationship. Almost always flagged.

Why it matters: US mobile carriers (T-Mobile, AT&T, Verizon) use attestation as the primary spam signal. A-attestation moves answer rates 30+ points vs B/C.

Full STIR/SHAKEN deep-dive →

BYOC (Bring Your Own Carrier)

The pattern of provisioning a real telecom carrier (DIDHub) and connecting it to a SaaS platform (Vapi, Retell, ElevenLabs, etc.) via standard SIP — rather than using the platform's bundled sandbox numbers.

Why it matters for AI voice agents:

• A-attestation signing (sandbox numbers usually carry B/C)
• True wholesale per-minute rates
• Real DIDs in 130+ countries
• Number portability across platforms

Full BYOC guide for AI agents →

Webhook

An HTTPS POST from DIDHub to your endpoint when something happens — an inbound SMS arrives, an outbound call's status changes, a number activates, a webhook event fires.

DIDHub webhooks:

• Sign every request with HMAC-SHA256 (X-DIDHub-Signature header)
• Retry with exponential backoff for up to 36 hours
• Carry a stable id field for idempotency

Full webhook reference →