OTP.ID can deliver one-time passwords through five distinct channels, each suited to different user environments and trust requirements. When you create an OTP transaction with POST /v3/request or POST /v3/send, you select a channel by setting the channel field in your request body. This page explains how each channel works, what field values to use, and what behaviours or restrictions apply.
Channel Summary
The table below gives a quick overview of all five channels before you dive into the details.
whatsapp_inbound is the only channel that cannot be used with POST /v3/send. It is exclusively available through POST /v3/request.
Channel Details
WhatsApp
SMS
Email
Missed Call
WhatsApp Inbound
WhatsApp (whatsapp)
OTP.ID sends an outbound WhatsApp message containing the one-time password directly to your user’s phone number. The message is delivered through OTP.ID’s WhatsApp Business integration and appears in the user’s WhatsApp inbox.channel field value: whatsappCompatible endpoints: POST /v3/request, POST /v3/sendRequired fields:
number — The destination phone number in international format, digits only, minimum 10 characters (e.g. 6281234567890).
Key behaviours:
- The OTP appears as a readable message in the user’s WhatsApp chat.
- Delivery depends on the user having an active WhatsApp account linked to the number.
- If delivery fails (e.g. the number is not registered on WhatsApp), the transaction status is set to
failed and your balance is not deducted.
SMS (sms)
OTP.ID delivers the one-time password as a standard SMS text message to the destination phone number. SMS works on virtually any mobile handset without requiring a data connection or a specific messaging application.channel field value: smsCompatible endpoints: POST /v3/request, POST /v3/sendRequired fields:
number — The destination phone number in international format, digits only, minimum 10 characters (e.g. 6281234567890).
Key behaviours:
- SMS is the most universally supported channel — it reaches feature phones, smartphones, and numbers without internet access.
- Delivery time depends on carrier routing and may be slower than WhatsApp in some regions.
- If the carrier cannot deliver the message, the transaction status is set to
failed and your balance is not deducted.
Email (email)
OTP.ID sends the one-time password to an email address. This channel is useful when your users authenticate by email rather than phone number, or when you want to support users who may not have a mobile number available.channel field value: emailCompatible endpoints: POST /v3/request, POST /v3/sendRequired fields:
number or email — Provide the recipient’s email address in either the number field or the dedicated email field.
Key behaviours:
- If you supply the email address in the
number field, OTP.ID treats it as the delivery destination.
- If you supply both
number and email, the email field takes precedence.
- Delivery depends on the email address being valid and the inbox being reachable (not full, not bouncing).
- If delivery fails, the transaction status is set to
failed and your balance is not deducted.
Or using the explicit email field:Missed Call (misscall)
OTP.ID places a call to your user’s phone number and immediately disconnects — no answer is required. The OTP is the last N digits of the calling number, where N equals your configured otp_length. The user reads the code from their missed-call log without needing to open any app or have internet access.channel field value: misscallCompatible endpoints: POST /v3/request, POST /v3/sendRequired fields:
number — The destination phone number in international format, digits only, minimum 10 characters (e.g. 6281234567890).
Key behaviours:
- The user does not need to answer the call — OTP.ID disconnects intentionally.
- The OTP is encoded in the caller ID itself, so there is no message to intercept.
- Works on any phone that displays a missed-call notification, including feature phones with no data connection.
- If the call cannot be placed (e.g. number unreachable), the transaction status is set to
failed and your balance is not deducted.
Missed Call is especially effective in markets where SMS delivery is unreliable or where users are sensitive to message-interception risks, since the code never travels as a text payload.
WhatsApp Inbound (whatsapp_inbound)
WhatsApp Inbound reverses the usual flow: instead of OTP.ID sending a message to your user, your user sends a WhatsApp message to OTP.ID’s central number. When you create a transaction with this channel, OTP.ID returns a token and pre-filled message template. Your application displays these to the user, who opens WhatsApp and sends the message. OTP.ID detects the incoming message, matches the token, and marks the transaction as verified.channel field value: whatsapp_inboundCompatible endpoints: POST /v3/request onlyRequired fields:
number — Optional. If you provide a phone number, OTP.ID only accepts a reply from that specific number (binding mode). If you omit number, OTP.ID accepts the first reply containing the matching token from any number.
Response — verification block:When you create a whatsapp_inbound transaction, the response includes a verification object with everything you need to guide your user:Your application must display this information to the user before the token expires.Example verification block in the response:whatsapp_inbound is the only channel that cannot be used with POST /v3/send. Attempting to use it with /v3/send will return a validation error.
Choosing the Right Channel
Not sure which channel to pick? Use this guidance:
- WhatsApp — Best default for markets with high WhatsApp adoption (e.g. Indonesia, Brazil, India). Rich, readable messages.
- SMS — Best fallback for users without WhatsApp or internet access. Maximum reach.
- Email — Use when your authentication flow is email-first or your users don’t provide a phone number.
- Missed Call — Use when SMS delivery is unreliable, or when you want a zero-text, caller-ID-based flow that works on feature phones.
- WhatsApp Inbound — Use when you want the user to initiate contact (e.g. opt-in flows, customer support contexts) or when outbound messaging templates are restricted.
For maximum resilience, consider requesting WhatsApp first and falling back to SMS if the transaction status returns failed. 
