# SIP routing methods

The routing method determines how DIDWW locates the customer SIP endpoint for
an inbound call. An inbound SIP trunk uses either **Static Endpoint** or
**Dynamic Registration**. Both methods deliver standard SIP call signaling.
They differ in how the destination is supplied and maintained.

## Routing method comparison

| Characteristic | Static Endpoint | Dynamic Registration |
| --- | --- | --- |
| Destination source | Host and optional port configured on the DIDWW trunk. | Contact address supplied by the customer system in SIP `REGISTER`. |
| Customer address | Normally stable and publicly reachable by DIDWW. | May change between successful registrations. |
| Authentication direction | DIDWW can optionally authenticate to the customer endpoint with SIP digest credentials. | The customer system authenticates its registration to DIDWW with the credentials generated for the trunk. |
| Destination maintenance | Updated in the User Panel or resolved from the configured hostname. | Refreshed by the customer system through repeated registration. |
| Multiple destinations | Supported through [DNS SRV](call-handling-and-delivery/service-failover-and-load-balancing.html#inbound-sip-failover-load-balancing) or by placing multiple trunks in a [Trunk Group](../trunk-groups/index.html). | Up to 10 simultaneous registrations are supported for one trunk. |
| Common use | PBX, SBC, or application with a fixed IP address or DNS hostname. | PBX or endpoint with a dynamic address, NAT, or registration-based connectivity. |

## Static Endpoint routing

With **Static Endpoint**, DIDWW constructs a SIP Request-URI from the trunk's
configured user part, host, and optional port. DIDWW then resolves the host as
needed and sends a SIP `INVITE` to that destination.

```
        %%{init: { "theme": "base", "themeVariables": { "actorBkg": "#e6f7ff", "actorBorder": "#1890ff", "noteBkgColor": "#fff7e6", "noteBorderColor": "#fa8c16" }}}%%
sequenceDiagram

   participant Caller as 📞 Caller
   participant DIDWW as DIDWW
   participant Endpoint as 🖥️ SIP endpoint
   Caller->>DIDWW: Calls the DID number
   DIDWW->>DIDWW: Select assigned trunk and resolve host
   DIDWW->>Endpoint: INVITE to configured destination
   Endpoint-->>DIDWW: SIP response
   DIDWW-->>Caller: Call proceeds or fails
```

### Destination construction

The Static Endpoint destination is the SIP Request-URI DIDWW places in the
outbound `INVITE`, built from these values:

- **User Part of R-URI** - Identifies the called resource. The `{DID}`
  placeholder inserts the called DID number in E.164 format. The resulting
  value appears before `@` in the SIP Request-URI.
- **Host** - The public IP address or DNS hostname of the receiving SIP system.
- **Port** - The destination signaling port. When present, DIDWW sends the call
  to that port after resolving the host. When empty, DIDWW performs DNS SRV
  discovery.

For example, a user part of `{DID}`, a host of `sip.example.com`, and a
port of `5060` produce a destination in this form:

```
sip:{DID}@sip.example.com:5060
```

The endpoint must be reachable on the selected transport and port and must
accept signaling from the relevant [DIDWW SIP signaling
addresses](sip-connectivity-reference.html#inbound-sip-connectivity-reference). These fields are configured under the
trunk's
[Static Endpoint settings](sip-trunk-settings-reference.html#inbound-sip-static-endpoint-fields).

### DNS resolution and multiple destinations

When a port is configured, DIDWW resolves a hostname through DNS address
records and sends the request to the resulting address. When the port is
empty, DIDWW uses DNS SRV records to discover multiple targets behind one host,
with failover between them within the same call attempt. For the SRV records,
target selection, and fallback behavior, see [DNS SRV
failover](call-handling-and-delivery/service-failover-and-load-balancing.html#inbound-sip-dns-srv-failover).

Static Endpoint is appropriate when the receiving service has a stable public
address or a DNS name maintained by the customer. If the endpoint cannot be
reached reliably at a single configured destination, use Dynamic Registration
or place multiple trunks in a [Trunk Group](../trunk-groups/index.html) for
failover.

### Preferred Server routing examples

The trunk's **Preferred Server** determines which DIDWW Point of Presence
delivers calls to the Static Endpoint. See [Preferred Server
values](sip-trunk-settings-reference.html#inbound-sip-preferred-server-values) for the available settings.

#### Specific Preferred Server

If you select a specific **Preferred Server** on a trunk, DIDWW delivers the
call through that Point of Presence even when another DIDWW location initially
receives the call. In the following example, the call reaches the Hong Kong
location and is forwarded through Frankfurt because the trunk uses **FRA**.

[![Inbound call received in Hong Kong and routed through the preferred Frankfurt Point of Presence](../../../_images/fig2.svg)](../../../_images/fig2.svg)


**Fig. 1.** Routing through a specific preferred server

#### Auto

With **Auto**, calls can be delivered from the DIDWW Point of Presence that
received them without being forwarded through a different location. DIDWW
can use an alternative Point of Presence when one becomes unavailable.

[![Inbound call delivered automatically through the DIDWW Point of Presence that received it](../../../_images/fig3.svg)](../../../_images/fig3.svg)


**Fig. 2.** Routing with Preferred Server set to Auto

## Dynamic Registration routing

With **Dynamic Registration**, the customer SIP system sends authenticated SIP
`REGISTER` requests to a DIDWW registrar. DIDWW stores the current Contact
address and uses it as the destination for calls assigned to that trunk. SIP
registration behavior is defined by [RFC 3261](https://www.rfc-editor.org/rfc/rfc3261#section-10).

```
        %%{init: { "theme": "base", "themeVariables": { "actorBkg": "#e6f7ff", "actorBorder": "#1890ff", "noteBkgColor": "#fff7e6", "noteBorderColor": "#fa8c16" }}}%%
sequenceDiagram

   participant Caller as 📞 Caller
   participant DIDWW as DIDWW
   participant Endpoint as 🖥️ SIP endpoint
   Endpoint->>DIDWW: REGISTER with credentials and Contact
   DIDWW-->>Endpoint: 200 OK
   Caller->>DIDWW: Calls the DID number
   DIDWW->>DIDWW: Select assigned trunk and active Contact
   DIDWW->>Endpoint: INVITE to registered Contact
   Endpoint-->>DIDWW: SIP response
```

### Registration and contact selection

The customer system registers using the username and password generated for
the inbound trunk. See [View SIP registration credentials](how-to-guides/view-sip-registration-credentials.html). It may
use the global
`sip.didww.com` registrar for automatic DNS-based load balancing between
locations or a documented
regional registrar. See [SIP Registrars](sip-connectivity-reference.html#inbound-sip-connectivity-registrars)
for the full list of registrar hostnames. The system must refresh the
registration before it expires so that the Contact remains available for
inbound routing. You can confirm active registrations in the
[Registration History](how-to-guides/view-registration-history.html).

A trunk supports up to **10 simultaneous registrations**. This permits more
than one active Contact to be associated with the trunk. DIDWW uses the active
registration state when selecting where to deliver a call. The customer
systems are responsible for maintaining their own registrations.

When **Use DID in R-URI** is enabled, DIDWW replaces the registered Contact's
user part with the called DID number in the inbound `INVITE`. This helps the
receiving system distinguish calls for different DID numbers that share the
same registered trunk. This setting is configured under the trunk's
[Dynamic Registration settings](sip-trunk-settings-reference.html#inbound-sip-dynamic-registration-fields).

Dynamic Registration is appropriate when the customer endpoint has a dynamic
public address, operates behind NAT, or is designed to receive calls through a
registration. If no valid registration is available, DIDWW has no current
Contact to which it can deliver the call through that trunk.

## Choosing a routing method

Choose **Static Endpoint** when:

- The receiving SIP system has a stable public IP address or DNS hostname.
- You want DIDWW to route to a destination explicitly configured on the trunk.
- You want to use customer-managed DNS SRV records for endpoint discovery,
  priority, and weight.

Choose **Dynamic Registration** when:

- The receiving system's public address can change.
- The system is behind NAT and maintains outbound registrations.
- Registration is the endpoint's normal method for receiving SIP calls.

Choose one method per trunk. To provide failover between different trunks or
between SIP and PSTN destinations, place those trunks in a
[Trunk Group](../trunk-groups/index.html) rather than treating the routing
methods as simultaneous settings on one trunk.

## Related resources

- [Create a SIP trunk](how-to-guides/create-sip-trunk.html) - Apply
  either routing method in the DIDWW User Panel.
- [SIP connectivity reference](sip-connectivity-reference.html) - Review signaling
  addresses and ports, RTP networks, codecs, DTMF, and encryption support.
- [Service Failover and Load Balancing](call-handling-and-delivery/service-failover-and-load-balancing.html#inbound-sip-failover-load-balancing) - Review
  DNS SRV, Trunk Group behavior, and routing-attempt CDRs.
- [Trunk groups](../trunk-groups/index.html) - Combine multiple trunks for
  failover and load balancing across trunks.
- [Configure Preferred Server routing](how-to-guides/configure-preferred-server-routing.html) - Enable or disable the
  DIDWW Points of Presence used for SIP call delivery.
- [Inbound Call Logs](../../../logs-analytics/call-logs/inbound-logs.html#inbound-cdr-logs) - Review routing attempts, SIP
  responses, and failover behavior for individual calls.

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