Inbound Call Redirection Methods#

Inbound calls can be redirected to an external phone number (PSTN) through your account’s Outbound Trunks. Advanced routing is achieved by responding to an incoming call with either a 30x Redirect SIP response or an in-dialog REFER request.


Before You Begin#

  • At least one Outbound Trunk is required, with Authentication method set to Credentials & IP-Based, and DIDWW SIP/RTP IP addresses allowed.

  • An Inbound SIP Trunk is required, with Max 30x Redirects or Max Transfers set to a value greater than 0 (configured in Advanced Signaling Settings).

  • Authentication is required on the Inbound SIP Trunk, using the same Auth User and Auth Password configured on the chosen Outbound Trunk.

  • The SIP destination URI is required in the format sip:NEW_PSTN_NUMBER@<outbound_endpoint> and included in the Contact header for 30x responses or the Refer-To header for REFER requests.

Note

  • The full list of outbound endpoints is available at Signaling Endpoints.

  • NEW_PSTN_NUMBER is the E.164 phone number, and <outbound_endpoint> is a DIDWW outbound endpoint (e.g., out.didww.com).


30x Redirect Method#

This method redirects an inbound call to a new destination without answering it.

  1. Your system replies to the DIDWW INVITE with a SIP 302 Moved Temporarily (or another 3xx response), including the new destination in the Contact header.

  2. DIDWW then starts a new outbound call to that SIP URI using your Outbound Trunk, authenticated with the credentials set on your Inbound Trunk.

Examples#

302 SIP Response

Example of a valid 302 Moved Temporarily response:

SIP/2.0 302 Moved Temporarily
Via: SIP/2.0/UDP 192.0.2.5:5060;branch=z9hG4bK-393929
From: <sip:CALLER_NUMBER@46.19.209.14>;tag=123456
To: <sip:DID_NUMBER@example.com>;tag=abcdef
Call-ID: 01-01-35531111-68BE960300095714-69CF836B
CSeq: 1 INVITE
Contact: <sip:NEW_PSTN_NUMBER@out.didww.com>
Content-Length: 0

For more details, see RFC 3261 Section 21.3 .

Call Flow Diagram
        %%{init: { "theme": "base", "themeVariables": { "actorBkg": "#e6f7ff", "actorBorder": "#1890ff", "noteBkgColor": "#fff7e6", "noteBorderColor": "#fa8c16" }}}%%
sequenceDiagram
  participant caller as Caller
  participant voice_in as DIDWW Inbound Trunk
  participant pbx as Your System
  participant voice_out as DIDWW Outbound Trunk

  caller->>voice_in: INVITE
  voice_in->>pbx: INVITE to your system<br/>sip:DID_NUMBER@example.com
  pbx-->>voice_in: 302 Redirect<br/>Contact: sip:NEW_PSTN_NUMBER@out.didww.com

  Note over voice_in: Redirect processing<br/>checks MAX_30x_REDIRECTS value
  voice_in->>voice_out: INVITE to your Outbound Trunk<br/>sip:NEW_PSTN_NUMBER@out.didww.com
  voice_out-->>voice_in: 401 Unauthorized + Auth Challenge

  Note over voice_in: Applies AUTH_USER and AUTH_PASSWORD from your Inbound Trunk

  voice_in->>voice_out: INVITE with Auth Header<br/>sip:NEW_PSTN_NUMBER@out.didww.com
  voice_out-->>voice_in: 200 OK
  Note over voice_in, voice_out: Call is established via Outbound Trunk
Use Cases

This method is ideal when a routing decision can be made immediately, without answering the call first.

  • Forward calls after hours to a service or mobile phone

  • Instantly route VIP callers to a direct line

  • Redirect desk calls to a mobile phone with follow-me rules


REFER Transfer Method#

This method transfers an inbound call to a new destination after it has been answered.

  1. Your system first answers the inbound call, creating an active SIP dialog.

  2. When a transfer is needed, your system sends an in-dialog SIP REFER request with the new destination in the Refer-To header.

  3. DIDWW validates the request against the MAX_TRANSFERS setting and replies with 200 OK.

  4. Your system then sends a BYE to close the original call leg. DIDWW initiates a new outbound call via your Outbound Trunk, authenticated with the credentials from your Inbound Trunk.

Examples#

SIP REFER Request

Example of a valid in-dialog REFER request:

REFER sip:CALLER_NUMBER@46.19.209.14 SIP/2.0
Via: SIP/2.0/UDP 192.0.2.5:5060;branch=z9hG4bK-443322
Max-Forwards: 70
From: <sip:USERPART@example.com>;tag=abcdef
To: <sip:CALLER_NUMBER@46.19.209.14>;tag=123456
Call-ID: 01-01-35531111-68BE960300095714-69CF836C
CSeq: 2 REFER
Refer-To: <sip:NEW_PSTN_NUMBER@out.didww.com>
Referred-By: <sip:DID_NUMBER@example.com>
Contact: <sip:DID_NUMBER@192.0.2.5:5060>
Content-Length: 0

For more details, see RFC 3515 .

Call Flow Diagram
Porting Request Flow
Use Cases

This method is useful when the call must first be answered before deciding where to route it:

  • Transfer calls after IVR menu selection (e.g., “Press 1 for Sales”)

  • Redirect from an AI voicebot to a live agent after intent detection

  • Forward calls from an agent’s softphone to an external specialist on the PSTN

Additional Resources#

Configure Call Transfers

Step-by-step guide on configuring inbound and outbound SIP trunks in DIDWW to enable call transfers using SIP 30x Redirect or SIP REFER signaling.

Configure Call Transfers