One of the first things to do after the keyword & short/long code configuration is complete for your account is to manage the default messaging for your program. These messages are sent to subscribers in various stages of opt-in and include messaging like welcome, already subscribed, invalid response, error, etc. 

Locate and configure default SMS messaging in the Contacts dashboard of your account. Click "Configuration", slide the toggle to "SMS", then click "Text Messaging":

Standard Messaging Options

"Send this message if you require a double opt-in to your SMS program"
Only use this optional message if double opt-in process is required for SMS subscribers from their mobile device, or if you are including SMS subscription in a website subscribe form. When configured, this will be the first message sent to the mobile device and requires a response.

"Send a welcome message to confirm the person's subscription"
This required message is usually the first message sent when a subscriber joins your SMS program. This message is required for the subscribe process from a mobile device.

"Send this message if the person was already subscribed"
This required message is sent when the subscriber's mobile number already exists. This message is required for the subscribe process from a mobile device.

"Send this message if the subscriber sends an invalid response"
This message is applicable to the the use of decision trees in Automation utilizing SMS messaging that requires a response from subscriber. Required if you plan to use SMS in automations that collect responses.

"Send this message if an unexpected error occurs"
This message is sent when some kind of error occurs during the sign-up process. This message is required for the subscribe process from a mobile device.

"Send a second welcome message, immediately after the first welcome message"
This optional message is sent immediately after the initial welcome message (above) to invite the subscriber to join your email list too.