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How to Build an SMS and Email Dual Opt-In Form

A dual opt-in form collects both an email address and a phone number in a single signup, giving you two channels to reach each subscriber. This approach lets you send longer content and offers by email while using SMS for time-sensitive alerts and reminders. Building both lists simultaneously from day one saves you from having to go back and collect phone numbers from email-only subscribers later.

Why Dual Opt-In Is Worth the Extra Field

Adding a phone number field to your signup form will reduce conversions compared to an email-only form, typically by 15% to 25%. That is a real tradeoff. But the subscribers you do capture are worth significantly more because you can reach them through two channels instead of one.

Email has higher engagement for content, newsletters, and detailed offers. SMS has near-100% open rates and is ideal for flash sales, appointment reminders, urgent updates, and time-limited promotions. Having both channels for the same contact means you can choose the right medium for each message instead of being limited to one. A restaurant might send the weekly specials menu by email but text a same-day lunch deal via SMS.

The math often works out in your favor despite lower form conversion. If an email-only form converts at 4% and a dual form converts at 3%, you lose 25% of signups but gain an SMS channel for every subscriber. Since SMS campaigns typically outperform email campaigns on immediate response rate, the revenue per subscriber from dual-channel contacts is usually higher than email-only contacts.

How to Build Your Dual Opt-In Form

Step 1: Design the form with both fields.
Include fields for email address, phone number, and optionally first name. Keep phone number as a required field if you want to build both lists equally, or make it optional with a note like "Add your phone number for exclusive text-only deals." On AI Apps API, both the Email Broadcast and SMS Broadcast apps share the same contact database in broadcastData, so a single form submission populates both channels.

Step 2: Add clear consent language.
Dual opt-in requires consent for both channels. Include text below the form that explains what the subscriber will receive on each channel. For example: "By signing up, you agree to receive marketing emails and text messages. Msg & data rates may apply. Reply STOP to opt out of texts. Unsubscribe from emails anytime." This language satisfies both CAN-SPAM requirements for email and TCPA requirements for SMS.

Step 3: Set up the form endpoint.
The form needs to submit both the email and phone number to your contact database. On the platform, the Lead Generation app handles multi-field form submissions, creating a single contact record with both email and phone fields populated. You can also use the Web Builder's newsletter form block with additional fields configured for phone number capture.

Step 4: Configure welcome messages for both channels.
Send a welcome email and a welcome SMS separately. The email delivers your lead magnet or welcome content. The SMS should be short and confirm the subscription: "Thanks for signing up, [Name]! You will get exclusive deals by text. Reply STOP to opt out." Each channel has different tone and length expectations, so write them independently rather than duplicating the same message.

Step 5: Set up separate unsubscribe handling.
A subscriber might want to opt out of SMS but keep receiving emails, or vice versa. Your system needs to handle each channel's opt-out independently. On AI Apps API, email unsubscribes and SMS STOP replies are processed separately through the suppression list and STOP handling systems, so opting out of one channel does not remove the subscriber from the other.

Compliance Considerations

Email Consent

Email marketing under CAN-SPAM requires an unsubscribe link in every email and a physical mailing address. You do not need explicit opt-in consent under CAN-SPAM (though best practice is to get it), but under GDPR you do need clear, affirmative consent for European subscribers. The form submission itself serves as consent if your language is clear about what the subscriber will receive.

SMS Consent

SMS marketing under TCPA requires express written consent before sending marketing text messages. This is a higher bar than email. The form must clearly state that the subscriber agrees to receive text messages, must include the message frequency, and must note that message and data rates may apply. The signed consent (form submission with this language visible) must be kept as a record. See the complete TCPA compliance guide for details.

Keeping Both Consents in One Form

You can collect consent for both channels in a single form as long as the disclosure language covers both. However, some businesses prefer to use separate checkboxes: one for email consent and one for SMS consent. This is not legally required in most cases, but it makes the consent more explicit and gives subscribers the choice to opt into one channel without the other from the start.

Important: Always store the timestamp and source of consent for SMS subscribers. If a carrier or regulator challenges your SMS sending, you need to prove that each subscriber explicitly agreed to receive text messages. The AI Apps API broadcastData table stores signup date and source automatically with each contact record.

Optimizing Dual Opt-In Conversion

Make phone optional first. Start with phone number as optional. Track what percentage of subscribers include it. If less than 30% add their phone, make the offer for SMS-specific value stronger.
Offer a channel-specific incentive. "Enter your phone number for a 10% off text" gives a reason to provide the phone number beyond what the email offer provides.
Use a two-step form. Collect the email on step one, then ask for the phone number on step two. This way you capture the email even if the visitor does not want to give their phone number. The multi-step funnel guide covers this technique in detail.
Show the value of each channel. Briefly explain what they will get by text versus email: "Email: Weekly tips and guides. Text: Flash sales and time-sensitive deals."

Build both your email and SMS lists at once with a dual opt-in form. Set it up in minutes.

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