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Short codes are 5- or 6-digit phone numbers designed for high-volume, application-to-person (A2P) messaging. They offer the highest throughput of any sender type — up to 1,000 messages per second — and are recognized by consumers as legitimate business numbers.

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When to use short codes

  • High-volume alerts: Time-sensitive notifications to large audiences
  • Two-factor authentication: OTP delivery at scale with high deliverability
  • Marketing campaigns: Promotional messages with keyword opt-in (e.g., “Text JOIN to 12345”)
  • Voting and polling: Interactive SMS campaigns
  • Emergency notifications: Critical alerts requiring maximum throughput

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Short code vs. other sender types

FeatureShort CodeToll-Free10DLC Long Code
ThroughputUp to 1,000 MPS20 MPSVaries by campaign
Setup time8-12 weeks1-2 weeksDays
CostHigher (monthly lease + per-message)ModerateLowest
Carrier trustHighestHighVaries by vetting score
MMS supportYesYesYes
Two-way messagingYes (keyword-based)YesYes
Vanity numbersAvailableNoNo

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Provisioning process

1

Request a short code

Navigate to Short Code in Mission Control and submit your request.Choose your type:
TypeDescriptionTimeline
RandomCarrier-assigned number8-10 weeks
VanityMemorable number you choose (e.g., 46835 = “GOULD”)10-12 weeks
Vanity codes are subject to availability. Request early — popular combinations may already be taken.
2

Complete your application

Provide details about your messaging program:
  • Company information — Legal name, address, contact
  • Use case description — What messages you’ll send
  • Message content samples — Representative examples
  • Volume estimates — Expected daily/monthly message volume
  • Opt-in flow — How users subscribe (web form, keyword, etc.)
  • Opt-out handling — STOP keyword support
  • Help response — HELP keyword response content
3

Carrier certification

Your application is submitted to each major US carrier for review and approval:
CarrierReview process
AT&TReviews content, opt-in flow, and compliance
T-MobileReviews content and message flow
VerizonReviews content and use case
US CellularReviews content
This is the longest step. Carrier certification typically takes 8-12 weeks. Each carrier reviews independently — you may get approved by some carriers before others. Plan your launch timeline accordingly.
4

Start messaging

Once approved by all target carriers, your short code appears under your messaging profile. Send messages using the same Messaging API as any other number type.

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Sending messages from a short code

Once provisioned, send messages the same way as any other sender type: 
curl -X POST https://api.telnyx.com/v2/messages \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -d '{
    "from": "12345",
    "to": "+15559876543",
    "text": "Your verification code is 847291. It expires in 5 minutes."
  }'
Short codes use ASCII 7-bit encoding by default (same character limits as GSM-7). See Message Encoding for details.

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Automated responses

Short codes must handle standard keywords to pass carrier certification. Telnyx manages these automatically.

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Required keywords

KeywordPurposeTelnyx handling
STOP (and UNSUBSCRIBE, END, QUIT, CANCEL)Opt-outAutomatic — user is blocked from receiving messages
HELP (and INFO)Help/support infoAutomatic — sends your configured help message
Campaign keywordCampaign opt-inAutomatic — sends your configured keyword response

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Customizing responses

After carrier certification, you can customize the HELP and campaign keyword responses:
Configure a custom help message that includes:
  • Your business name
  • What messages the user is subscribed to
  • How to get support (phone number or email)
  • How to opt out
Example:
Acme Corp: You're subscribed to order updates. For help, call 1-800-555-0123 
or email support@acme.com. Reply STOP to unsubscribe. Msg&data rates may apply.
When a user texts your campaign keyword (e.g., “JOIN”), they receive a confirmation message.Example:
Welcome to Acme Corp alerts! You'll receive order updates and promotions. 
Reply HELP for help, STOP to cancel. Msg&data rates may apply. ~4 msgs/month.
After certification, you can disable automatic responses for HELP and campaign keywords to handle them yourself via webhook. Contact support@telnyx.com to request this change.
STOP responses cannot be disabled. Opt-out handling is managed at the carrier level. Once a user opts out, they are blocked from receiving messages — you cannot send a custom opt-out confirmation.

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Use case examples

Keyword: N/A (API-triggered) Flow: User initiates login → your app sends OTP via short code → user enters code
Your Acme verification code is 847291. It expires in 5 minutes. 
If you didn't request this, ignore this message.
Why short code: Highest deliverability, fastest delivery, trusted by carriers.
Keyword: JOIN Flow: User texts JOIN to 12345 → receives welcome message → gets promotional messages
Welcome msg: Acme Corp: Thanks for joining! Get exclusive deals and updates. 
Reply HELP for help, STOP to cancel. Msg&data rates may apply. ~4 msgs/month.

Promo msg: Acme Flash Sale! 30% off all items today only. Shop now: https://acme.com/sale
Reply STOP to opt out.
Keyword: ALERT Flow: User opts in → receives critical alerts (weather, school closings, etc.)
ALERT: Severe thunderstorm warning for Springfield County until 8 PM. 
Seek shelter immediately. Updates at https://acme.com/alerts
Why short code: Maximum throughput (1,000 MPS) for time-critical mass notifications.

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Carrier certification requirements

Failing to meet these requirements will delay or prevent certification. Prepare these items before submitting your application.
RequirementDetails
Opt-in mechanismClear, documented method for users to subscribe (web form, keyword, paper form)
Opt-in disclosureUsers must know what they’re subscribing to, message frequency, and that rates may apply
Opt-out supportMust honor STOP and similar keywords immediately
Help supportMust respond to HELP with business name, support contact, and opt-out instructions
Message frequencyMust disclose expected message frequency at opt-in
Content complianceMessage content must match the registered use case
Privacy policyPublished privacy policy covering SMS data collection
Terms of servicePublished terms covering messaging program

Choosing Your Sender Type

Compare short codes, toll-free, long codes, and alphanumeric sender IDs.

Rate Limiting

Short code throughput limits and queuing behavior.

Advanced Opt-In/Out

Customize opt-in and opt-out behavior.

Message Encoding

Understand ASCII 7-bit encoding used by short codes.