How SMS service works#

DIDWW SMS lets you receive text messages on your DID numbers and send messages from your applications. SMS trunks connect DIDWW to your systems, and Sender ID Verifications approve the Sender IDs used for application-to-person (A2P) messaging.

How SMS is sent and received#

Every message travels through an SMS trunk. The trunk type defines how DIDWW and your systems exchange messages: by email, over HTTP, or over SMPP.

Trunk type	Direction	How messages are exchanged
SMS to Email	Inbound	DIDWW forwards each inbound SMS to an email address.
HTTP IN	Inbound	DIDWW sends each inbound SMS to your HTTP endpoint as an HTTP request.
HTTP OUT	Outbound	Your application submits SMS to the Outbound SMS API. DIDWW sends status callbacks to the trunk’s Callback URL.
SMPP ESME	Inbound and outbound	Your application binds to the DIDWW SMPP server to send and receive SMS.
SMPP SMSC	Inbound and outbound	DIDWW binds to your SMSC to deliver and receive SMS.

Inbound SMS#

When someone sends a text message to your DID number, the originating network delivers the message to DIDWW. DIDWW delivers it through the SMS trunk or SMS trunk group assigned to that DID number.

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sequenceDiagram
   autonumber
   participant sender as Sender
   participant network as Mobile network
   participant didww as DIDWW
   participant system as Your system

   sender->>network: Send SMS to DID number
   network->>didww: Deliver SMS to DIDWW
   Note over didww: Find the SMS trunk or trunk group<br/>assigned to the DID number
   didww->>system: Deliver SMS by email, HTTP, or SMPP

Outbound SMS#

Your system submits a message to DIDWW through an HTTP OUT, SMPP ESME, or SMPP SMSC trunk. DIDWW checks the trunk and the sender, routes the message to the destination network, and reports the result.

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sequenceDiagram
   autonumber
   participant app as Your system
   participant didww as DIDWW
   participant network as Destination network
   participant recipient as Recipient

   app->>didww: Submit SMS through an SMS trunk
   Note over didww: Check the trunk<br/>and the sender
   didww->>network: Route SMS
   network->>recipient: Deliver SMS
   didww-->>app: Report the result

DID numbers, trunks and sender IDs#

DID numbers receive inbound SMS. A DID number can receive SMS when it supports the Inbound SMS feature. To receive messages, assign an SMS trunk or an SMS trunk group to the DID number. Several DID numbers can use the same trunk.

SMS trunks connect DIDWW to your systems. An inbound message is delivered through the trunk assigned to the DID number that received it. An outbound message is sent through the trunk that your application uses. Outbound SMS does not require a DID number to be assigned to the trunk.

SMS trunk groups combine several SMS trunks. When a DID number uses a trunk group, DIDWW attempts the member trunks in priority order. If a delivery attempt fails, DIDWW tries the next member trunk in priority order. See SMS trunk groups.

Sender IDs define who an outbound message comes from:

For P2P messaging, the sender is a DID number that supports the Outbound P2P SMS feature. The trunk’s Source Address Settings control which of these DID numbers it can use. By default, a trunk can use every DID number with Outbound P2P SMS enabled in your account.
For A2P messaging, the sender is a Sender ID covered by an active Sender ID Verification, such as an Alphanumeric, Long-code, or Toll-Free Sender ID. Messages must be sent through the outbound SMS trunk associated with the verification.

P2P and A2P messaging#

DIDWW supports two types of outbound messaging traffic:

Type	Description	Sender
Person-to-person (P2P)	Low-volume, two-way exchange of SMS between end users.	A DID number with the Outbound P2P SMS feature.
Application-to-person (A2P)	High-volume SMS generated by applications and sent to subscribers.	A Sender ID registered through an active Sender ID Verification.

P2P and A2P messages have separate rates. Download the Outbound P2P SMS Trunk Rates and Outbound A2P SMS Trunk Rates pricelists from SMS > SMS Trunks in the DIDWW User Panel, or see Download pricelists. In the Outbound SMS logs, you can filter messages by service type.

Inbound A2P messages sent to DID numbers can be restricted in some countries. See A2P messaging in the Inbound SMS Guidelines.

Service access and message delivery#

Which messages you can receive and send depends on the features of your DID numbers and on your Sender ID Verifications:

Inbound SMS requires a DID number that supports Inbound SMS and an assigned SMS trunk or trunk group.
Outbound P2P SMS requires a DID number that supports Outbound P2P SMS.
Outbound A2P SMS requires an active Sender ID Verification associated with the outbound SMS trunk.

DID numbers are optimized to receive SMS within their country of registration. For delivery considerations, see the Inbound SMS Guidelines.

P2P Outbound SMS access#

P2P Outbound SMS is enabled automatically after your DIDWW account is verified through Account Setup. You do not need to contact DIDWW Sales or Support to request access. Account verification starts when you first buy a DID number. After that, you can send P2P SMS from any DID number in your account that supports Outbound P2P SMS, through an outbound SMS trunk that allows that number as the sender.

Submission and delivery#

When DIDWW accepts a message, the message is not yet delivered. Delivery is reported separately:

Outbound SMS API: A 201 Created response confirms that DIDWW accepted the request and returns the message ID. DIDWW then sends a status callback with the processing result to the trunk’s Callback URL. When enabled, a delivery receipt (DLR) event reports the final delivery status from the carrier. See Status callbacks and delivery receipts.
SMPP: Each PDU is acknowledged, for example submit_sm with submit_sm_resp. Delivery receipts are sent in deliver_sm. See SMPP message flow.
HTTP IN: If your endpoint does not respond with a 2xx status code, DIDWW retries delivery until the message’s time to live (TTL) of 3600 seconds expires. See Technical details.

Review sent and received messages in the Outbound SMS logs and Inbound SMS logs.

Message length and billing#

SMS messages are billed per fragment. A long message is split into several fragments, and its encoding determines how many characters fit in each fragment. Each fragment is billed separately. For character limits, concatenation, and billing examples, see the SMS encoding reference.