Double optin is advised but not required. It's outlined as a guideline for use with shortcode primarily.
You can read the entire guidelines here https://support.twilio.com/.../223134707-Industry...
The core key compliance requirement is ensuring that the recipients of your text messages (your recipients) have expressly consented or “opted-in” to receiving text messages as part of your campaign.
There are examples more specifically of opting in via phone versus web app:
This is why we send out the initial compliance text conveying what it's for and they may receive (insert frequency amount) of sms messages per month.