Short message service (SMS)

Short message service (SMS) is the text-message protocol used on mobile networks to send short text messages between phones. Standardized in the 1980s and rolled out commercially in the 1990s, SMS is one of the longest-lived and most universally supported communication channels in the world. Despite the rise of richer messaging apps, SMS remains a default channel for customer notifications, two-factor authentication, and increasingly conversational customer support.

A standard SMS message is limited to 160 characters of GSM-7 encoding (or 70 characters in Unicode). Longer messages are split into segments and reassembled on the receiving device.

How SMS works

SMS messages travel over the same cellular signaling channels used to set up phone calls, which is why they can be delivered even when data service is unavailable. When a phone sends an SMS, the message is routed to a Short Message Service Center (SMSC) operated by the carrier, which queues and forwards the message to the recipient's carrier and ultimately to the recipient's device. Delivery is best-effort but reliable in practice — the vast majority of messages arrive within seconds.

For businesses, SMS is typically sent through a programmatic API provided by carriers or aggregators (Twilio, Bandwidth, Sinch, MessageBird). Outbound business SMS in the U.S. uses one of three sender types: 10DLC (10-digit long codes), toll-free numbers, or short codes — each with different throughput, deliverability, and compliance characteristics.

SMS vs. MMS vs. RCS

These three protocols often get lumped together but differ substantially.

• SMS: Text only, 160-character segments, universal across all mobile devices, no read receipts, no rich media.
• MMS (multimedia messaging service): Extends SMS to support images, audio, video, and longer text. Higher cost per message and less universally consistent.
• RCS (rich communication services): A modern protocol that adds chat-like features — read receipts, typing indicators, rich cards, suggested replies, and branded sender profiles. Supported on most Android devices and now on iOS as of 2024.

For business messaging, RCS is rapidly emerging as the upgrade path for customer experience, but SMS remains the universal fallback when RCS isn't supported.

SMS in customer experience

SMS is used heavily across the customer lifecycle for one simple reason: open rates are extraordinary. Industry benchmarks put SMS open rates above 95%, with most messages read within minutes. Common use cases include order and shipping notifications, appointment reminders, two-factor authentication codes, marketing offers, and increasingly two-way conversational support. SMS is the default escalation channel from in-app and web channels because customers reliably see it.

SMS as a support channel

Two-way SMS has become a core part of modern omnichannel customer support. Customers reply to a delivery notification with a question; the response is routed into the contact-center stack alongside chat and email; an AI agent or human agent picks it up. SMS as a support channel works best when paired with a unified routing layer — typically a CCaaS platform — so customer history follows the conversation across channels. An AI-powered conversational AI agent handling SMS can resolve a meaningful share of inbound questions before they ever reach a human.

SMS compliance and regulation

SMS marketing and messaging are heavily regulated. In the U.S., the Telephone Consumer Protection Act (TCPA) requires explicit consent before businesses can send marketing SMS, with statutory damages of $500–$1,500 per unsolicited message. CTIA carrier guidelines layer on additional rules around opt-in, opt-out language (STOP, HELP), throughput, and prohibited content. The FCC's telemarketing and robocall page is the canonical reference for U.S. SMS compliance. Outside the U.S., similar consent regimes exist under GDPR, CASL, and country-specific laws.

Frequently asked questions

What does SMS stand for? SMS stands for short message service — the standardized protocol for sending short text messages over mobile networks.

What is the difference between SMS and a text message? They're effectively the same thing. Text message is the everyday term; SMS is the technical protocol name. Strictly, a text message can also refer to an MMS or RCS message, while SMS specifically means text-only.

What is the difference between SMS and MMS? SMS is text-only and limited to 160 characters per segment. MMS supports images, audio, video, and longer text but costs more and has less consistent device support.

What is the character limit for SMS? 160 characters when using standard GSM-7 encoding, or 70 characters when using Unicode (for emojis or non-Latin alphabets). Longer messages are split into concatenated segments.

Is SMS still relevant for business? Yes — SMS open rates remain above 95%, and it's the most universally supported customer channel. It's heavily used for transactional notifications, two-factor authentication, and two-way support.

For a deeper dive, download Decagon's guide to agentic AI for customer experience.