# SIP trunks

An inbound **SIP trunk** is a logical connection that delivers calls from DIDWW
to a SIP endpoint, such as an IP PBX, session border controller (SBC),
communications platform, or application server, using the [Session Initiation
Protocol (SIP)](https://www.rfc-editor.org/rfc/rfc3261). Use an inbound SIP
trunk when calls to DID numbers on your DIDWW account must be delivered to a
SIP endpoint. The trunk defines how DIDWW routes SIP signaling to the
destination and provides settings for controlling signaling, media, and call
handling.

## How SIP trunks work

When a caller dials a DID number, the public telephone network routes the call
to DIDWW. DIDWW uses the trunk assigned to the DID number to determine how the
call should be delivered. For an inbound SIP trunk, DIDWW determines the SIP
destination from the trunk configuration and sends a SIP `INVITE` to the SIP
endpoint.

When the call is answered, RTP carries the media between DIDWW and the SIP
endpoint.

```
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  "themeVariables": {
    "fontSize": "14px",
    "primaryColor": "#e0f2fe",
    "primaryBorderColor": "#38bdf8",
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}}%%

flowchart LR

   caller["Caller"] --> |dials DID| did["DID number"]
   did --> |assigned trunk| trunk["Inbound SIP trunk"]
   trunk --> |SIP INVITE| endpoint["Customer SIP endpoint"]

   classDef endpoint fill:#e0f2fe,stroke:#0066cc,color:#1f2d3d,stroke-width:1.5px
   classDef process fill:#ccfbf1,stroke:#2dd4bf,color:#1f2d3d,stroke-width:1.5px

   class caller,endpoint endpoint
   class did,trunk process

   linkStyle default stroke:#0066cc,stroke-width:1.5px
```

## SIP trunks and DID numbers

A DID number and an inbound SIP trunk serve different purposes in DIDWW. The
DID number receives inbound calls, while the SIP trunk defines how DIDWW
delivers those calls to your SIP endpoint.

A DID number must be [assigned to an inbound
trunk](../../../phone-numbers/my-numbers/how-to-guides/assign-voice-trunk.html#user-panel-assign-trunk) before calls can
be delivered through that trunk.
Multiple DID numbers can be assigned to the same SIP trunk, allowing calls to
those numbers to use the same SIP configuration.

The DID number must have sufficient [Capacity](../../../phone-numbers/capacity/how-capacity-works.html) to receive
concurrent calls. The
trunk's **Capacity Limit** setting can further restrict the number of
simultaneous calls delivered through that trunk, but it cannot add DID
Capacity.

## Routing methods

The routing method determines how DIDWW obtains the SIP destination for calls
delivered through the trunk. Each inbound SIP trunk uses one of the following
routing methods:

- **Static Endpoint** - DIDWW sends calls to a SIP destination configured on
  the trunk. Use this method when your SIP endpoint has a stable, publicly
  reachable address.
- **Dynamic Registration** - Your SIP system registers with DIDWW, and DIDWW
  sends calls to a destination learned through the registration. Use this
  method when your system uses registration-based connectivity, such as when
  its public address can change or it operates behind NAT.

For a detailed comparison and information about destination selection, see
[SIP routing methods](sip-routing-methods.html).

## Failover and redundancy

Inbound SIP delivery can use several mechanisms to improve redundancy at
different stages of the routing process:

- **Preferred Server** controls the DIDWW Point of Presence (PoP) used to
  deliver calls to the SIP endpoint. The default **Auto** option allows DIDWW
  to select the appropriate PoP for call delivery.
- **DNS SRV** can provide multiple SIP destinations for the hostname configured
  for a Static Endpoint.
- **Trunk Groups** combine multiple SIP or PSTN trunks and provide failover and
  load balancing between separate routing destinations.

These mechanisms serve different purposes and can be used together as part of
a resilient inbound routing configuration. For SIP destination selection, see
[SIP routing methods](sip-routing-methods.html). For detailed failover and
load-balancing behavior, see [Service Failover and Load
Balancing](call-handling-and-delivery/service-failover-and-load-balancing.html#inbound-sip-failover-load-balancing).

---

## Get started

Create a trunk and prepare your SIP endpoint for inbound calls.

**Create a SIP trunk**

Create and initially configure an inbound SIP trunk in the DIDWW User Panel.

[Create a SIP trunk](how-to-guides/create-sip-trunk.html)

**SIP routing methods**

Compare Static Endpoint and Dynamic Registration and how each determines
the call destination.

[SIP routing methods](sip-routing-methods.html)

**SIP connectivity**

Understand signaling, media, authentication, and firewall requirements
for your SIP endpoint.

[SIP connectivity](sip-connectivity.html)

## Manage trunks and routing

View and update existing trunks and routing settings.

**Manage SIP trunks**

View, edit, or delete SIP trunks and enable CNAM lookup.

[Manage SIP trunks](how-to-guides/manage-sip-trunks.html)

**Configure Preferred Server routing**

Choose which DIDWW Points of Presence are available for trunks using Auto.

[Configure Preferred Server routing](how-to-guides/configure-preferred-server-routing.html)

## Dynamic Registration

These guides apply only to trunks using Dynamic Registration.

**View SIP registration credentials**

Find the username, password, and registrar endpoint needed to register
your SIP system.

[View SIP registration credentials](how-to-guides/view-sip-registration-credentials.html)

**View registration history**

Review successful registration intervals and the endpoints registered
to your trunk.

[View registration history](how-to-guides/view-registration-history.html)

## Call handling and delivery

Explore optional features for resilient delivery, transfers, and fax.

**Configure call transfers**

Configure your trunks and SIP platform to redirect or transfer inbound
calls through an outbound SIP trunk.

[Configure call transfers](how-to-guides/configure-call-transfers.html)

**Service failover and load balancing**

Understand DIDWW network redundancy and delivery through DNS SRV and
trunk groups.

[Service failover and load balancing](call-handling-and-delivery/service-failover-and-load-balancing.html)

**Fax support**

Check the requirements for receiving fax calls over SIP using T.38.

[Fax support](call-handling-and-delivery/fax-support.html)

## Call information

Learn what caller information DIDWW can deliver to your SIP endpoint.

**CNAM IN for SIP trunks**

Review caller-name delivery for U.S. caller IDs, availability, and
lookup charges.

[CNAM IN for SIP trunks](call-information/cnam-in.html)

**STIR/SHAKEN for inbound calls**

Understand the caller authentication headers DIDWW delivers to your
SIP endpoint.

[STIR/SHAKEN for inbound calls](call-information/stir-shaken-headers.html)

**Calling Party Category (CPC)**

Identify the caller's network or device category using information
delivered in SIP requests.

[Calling Party Category (CPC)](call-information/cpc-usage.html)

## Technical references

Look up configuration fields and connectivity specifications.

**SIP trunk settings reference**

Find field definitions, defaults, supported values, and limits for
SIP trunk settings.

[SIP trunk settings reference](sip-trunk-settings-reference.html)

**SIP connectivity reference**

Find signaling and media addresses, ports, codecs, DTMF modes, and
encryption options.

[SIP connectivity reference](sip-connectivity-reference.html)

## Related resources

Assign your trunks to numbers and explore additional routing controls.

**Assign a voice trunk**

Assign an inbound trunk or trunk group to one or more DID numbers.

[Assign a voice trunk](../../../phone-numbers/my-numbers/how-to-guides/assign-voice-trunk.html#user-panel-assign-trunk)

**Trunk groups**

Combine SIP and PSTN trunks for failover and load balancing.

[Trunk groups](../trunk-groups/index.html)

**Number lists**

Allow or reject calls by caller number, prefix, or number length.

[Number lists](../number-lists/index.html)

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