SMS interrupts — it lands on a lock screen with a sound or vibration regardless of whether the recipient wants it right now, unlike email sitting quietly in an inbox. Every rule here follows from that one fact: SMS is the highest-cost-to-the-recipient channel available, and it should be reserved for messages that actually justify an interruption.

Only urgent or actionable content

Send SMS only for messages that are genuinely time-sensitive or require prompt action: a one-time passcode, a delivery arriving in the next hour, a security alert, an appointment reminder for tomorrow — not general updates, newsletters, or anything that could wait for email.
Ask, for every SMS before it ships: would the recipient be annoyed to be interrupted by this specific message right now? If the honest answer is yes, it belongs in email or in-app notification instead, not SMS.
Never use SMS as a secondary marketing channel just because deliverability is higher than email — higher deliverability for unwanted content is a liability, not an opportunity; it makes opt-outs and complaints correspondingly more likely and more damaging.

Explicit opt-in and easy STOP

Require explicit, affirmative opt-in specifically for SMS (not inherited from an email or account signup checkbox) — most jurisdictions and carrier rules require this, and beyond compliance, unrequested SMS generates disproportionate complaint rates compared to unrequested email.
Support and honor "STOP" (and common variants) as an immediate, unconditional opt-out with no additional steps — a delayed or non-functional STOP reply is both a compliance violation and a fast path to a carrier filtering the sending number entirely.
Confirm opt-in and opt-out with a short reply message so the recipient has explicit confirmation their preference took effect, not silent uncertainty about whether STOP actually worked.

Sender identification

Identify the sender by name in the first message of any conversation, or in every message if using a shared/shortcode number the recipient won't recognize — an SMS from an unrecognized number with no sender name reads as spam or a scam attempt and gets ignored or reported.
Keep the sender identity consistent across all messages in a relationship (same number or same named sender) — a recipient who gets messages from different numbers for the same service can't build the pattern recognition that makes them trust and open future messages.

160-character discipline

Write to fit within a single 160-character segment wherever possible — messages beyond that split into multiple segments (billed and often displayed as multiple messages), and long SMS reads as a channel mismatch; if the content needs more space, it likely belongs in email or a linked page.
Front-load the essential fact in the first sentence — assume the recipient reads only the first line in a notification preview before deciding whether to open the full message.

Quiet hours

Never send non-emergency SMS outside a reasonable local-time window (typically not before ~8am or after ~9pm in the recipient's timezone) — an SMS notification arriving at 3am for anything short of a genuine emergency damages trust in the sender regardless of the message's actual content.
Respect the recipient's actual local timezone, not the sender's — a scheduled send that's daytime for the business but the middle of the night for the recipient is still a quiet-hours violation.

Link shortening with trust

Use a branded or recognizable short domain rather than a generic, unbranded shortener — an unrecognizable short link in an SMS is a top phishing red flag, and recipients trained to distrust random-looking links will ignore legitimate ones too.
Never send a link that requires the recipient to enter sensitive credentials on the resulting page without independent context confirming the message is genuinely from the expected sender — SMS phishing (smishing) specifically exploits the same trust this rule set is trying to build responsibly.

Fallback channels

Provide a fallback channel (email, in-app) for the same notification when SMS delivery fails or when the recipient hasn't opted in — don't let a message that matters (an OTP, a critical alert) depend on SMS alone as a single point of failure.
Let recipients choose their preferred channel per notification type where the product supports multiple channels, rather than defaulting everyone into SMS for everything.

Cost awareness

Remember that SMS has a real per-message carrier cost, unlike email — batch or deduplicate notifications where a single message can carry multiple updates instead of firing one SMS per individual event.
Monitor delivery cost against actual engagement value per notification type, and demote low-value notification types out of SMS and into a cheaper channel when data shows they're not earning the cost of the interruption.

SMS Notification Rules