# Inbound SMS Guidelines The Inbound SMS guidelines outline important considerations when receiving SMS on DID numbers. These guidelines explain how delivery works, highlight potential restrictions, and describe the limitations related to A2P messaging, SMS verification, and troubleshooting. --- ## SMS Delivery DID numbers are optimized to receive SMS **within their country of registration**. International delivery may be affected by: > - **Local restrictions** - Some countries enforce regulatory or technical limits on inbound international SMS to DIDs. > - **Outdated routing data** - Originating networks may rely on outdated or incomplete routing databases, causing delivery failures. Note Delivery behavior may vary depending on the **originating network** and **message type**, and international traffic can sometimes experience **delays or message loss** due to upstream carrier dependencies. --- ## SMS to Email Delivery If you are using an [SMS to Email trunk](../create-sms-to-email-trunk.html#sms-in-email), inbound SMS messages will be automatically forwarded to the **recipient email address** you configured. > - All forwarded messages are delivered **from** the address `sms-forwarding@didww.com`. > - The **subject line** and **email body** are generated according to the configuration you set when creating the trunk. > - Supported placeholders (e.g., `{SMS_SRC_ADDR}`, `{SMS_DST_ADDR}`, `{SMS_TEXT}`) can be used in the trunk setup to customize the email output. Note Make sure your email system allows incoming mail from `sms-forwarding@didww.com` so that inbound SMS messages are not filtered as spam or rejected. --- ## SMS Verification Notice Using DID numbers for **SMS verification** is **not recommended**. DID numbers are **non-exclusive** and may be reassigned over time, which can create security and continuity risks for verification workflows. Note - Some applications and platforms **may not deliver** one-time codes to DID numbers by policy. They may restrict delivery to mobile (cellular) ranges only, or block numbers identified by number-validation checks. - If you must use SMS verification, ensure you have an alternative verification path and do **not** rely on a single DID for long-term account recovery. --- ## A2P Messaging DID numbers are primarily intended for two-way person-to-person communication. In certain countries, Application-to-Person (A2P) traffic to DIDs may be restricted by regulation or commercial policy. Note To reduce costs, some originators run **number validation** before sending messages. These tools can misclassify DID prefixes if their databases are outdated or incomplete. Originators may also deliver A2P messages with **alphanumeric sender IDs**, which can behave differently from numeric MSISDNs. --- ## Encoding and Concatenation Inbound SMS may arrive in different encodings depending on the sender: > - **GSM-7** - Standard encoding, up to 160 characters. > - **UCS-2 (Unicode)** - Required for non-Latin scripts (e.g., Chinese, Arabic, Cyrillic), up to 70 characters per message. When a message exceeds these limits, it is delivered as a **concatenated message** (multi-part SMS). A **User Data Header (UDH)** is used to split and reassemble long messages. Up to **255 parts** may be supported for a single message. Note Applications must be able to process UDH headers and reassemble multi-part SMS to correctly display long messages. --- ## Firewall Configuration To receive inbound SMS correctly, ensure your firewall allows traffic from DIDWW inbound SMS delivery IP addresses. Requests will always originate from DIDWW-defined IPs (see [Endpoints](smpp-specification.html#smpp-endpoints)), which must be explicitly permitted. --- ## Troubleshooting If you have delivery issues after reviewing the points above, contact **DIDWW Technical Support** at [support@didww.com](mailto:support%40didww.com) and include the SMS details below. | **Field** | **Description** | | --- | --- | | **Date and Time (UTC)** | Exact timestamp when the SMS was sent. | | **Originating Number** | The sender's phone number. | | **Destination DID** | The DID number that should receive the SMS. | | **Originating Network/Operator** | Name of the operator or platform that submitted the SMS, if known. | | **Any Error/Status from Sender Side** | Delivery status, error code, or logs provided by the originating platform. | ### SMS Details Example When opening a trouble ticket to our support team at [support@didww.com](mailto:support%40didww.com) you may copy the example below and replace the placeholder values with your real SMS details. ``` Date and Time (UTC): YYYY-MM-DD HH:MM:SS Originating Number: + Destination DID: + Originating Network/Operator: Any Error/Status from Sender Side: ``` Note More context reduces investigation time. If you performed **multiple test attempts**, please share **multiple samples** with different timestamps. On this page