If you want your clients to Opt-in to text before you begin your conversations, you can enable the Double Opt-in feature in your account to send automated opt-in messages.  
This is an organization wide feature that will apply to your entire team's settings and every contact will have to opt-in before you can start texting. 

Opting In & Opting Out

1. Before you begin a conversation with a contact, you'll first send an automated confirmation text.

2. After your contact opts in,  you can start composing your message.

3. If your contact responds "stop" they will automatically be opted-out and you won't be able to send texts to the contact until they opt-in again.

To enable or disable this feature, navigate to settings> organization settings > double Opt-in

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