SMS is a core channel of the Sinch Conversation API. The API handles SMS-specific details like encoding detection and message concatenation automatically.

Prerequisites

A service plan with at least one virtual number assigned.
A Conversation API app created in the same region as your service plan.

SDK Reference

Character Encoding

The API auto-detects encoding based on message characters:

Encoding	Max chars per SMS	Max chars per part (multipart)
GSM 7-bit	160	153
UCS-2 (Unicode)	70	67

GSM 7-bit covers standard Latin characters, digits, and common symbols.
Any character outside GSM 7-bit (accented chars, CJK, emoji) triggers UCS-2, halving capacity.
A single emoji forces the entire message to UCS-2.

Auto Encoding

Reduces message parts by transliterating special characters (e.g., smart quotes to straight quotes). Emojis and CJK characters are not converted. Contact your Sinch account manager to enable.

Concatenated Messages

When a message exceeds single-SMS limits, it is split into parts. Each part includes a UDH that reduces usable characters. Control max parts with SMS_MAX_NUMBER_OF_MESSAGE_PARTS.

SMS Channel Properties

Set under channel_properties in your message request:

Property	Description
`SMS_SENDER`	Sender number or alphanumeric sender ID
`SMS_MAX_NUMBER_OF_MESSAGE_PARTS`	Max concatenated parts allowed (integer)
`SMS_FLASH_MESSAGE`	Whether this is a flash SMS message

Sender ID Types

Type	Description	Example
Long code	Standard phone number	`+15551234567`
Short code	5-6 digit number	`12345`
Alphanumeric	Brand name (1-way only)	`MyBrand`
Toll-free	Toll-free number	`+18001234567`
10DLC	US registered local number	`+15551234567`

Opt-Out Handling

Opt-out keywords (STOP, UNSUBSCRIBE, etc.) can be processed by Sinch automatically for US/Canada numbers when consent management is active.
Inbound opt-out messages are delivered via webhook as Mobile Originated (MO) messages.
You must honor opt-outs and maintain your own suppression list for compliance.
Re-opt-in typically requires the user to send a keyword like START.

Message Examples

Send SMS / Fallback patterns — See Messages API Reference. Set channel to SMS and include SMS_SENDER in channel_properties. For fallback, add channel_priority_order array.
Inbound SMS handling — See MESSAGE_INBOUND trigger. Opt-out keywords (STOP) arrive as contact_message.text_message.

Code Examples

SMS Gotchas

Encoding surprises. A single non-GSM character forces UCS-2 encoding, doubling message parts. Sanitize input or enable Auto Encoding.
Sender ID rules vary by country. Alphanumeric sender IDs are not supported in the US or Canada. Some countries require pre-registered sender IDs.
10DLC registration is required. US A2P messaging over local numbers requires 10DLC brand and campaign registration. Unregistered traffic will be filtered.
Short code limitations. US short codes require dedicated provisioning and carrier approval. Cannot send MMS via Conversation API.
Concatenation costs. Each SMS part is billed separately. A 161-character GSM message costs 2 SMS credits.
Opt-out compliance. US/Canada regulations (TCPA, CASL) require honoring opt-outs. Sinch handles standard keywords automatically when consent management is active.
Delivery receipts are not guaranteed. Some carriers do not return delivery receipts. Handle UNKNOWN status gracefully.