Marketing > SMS > Contacts opens the SMS Contacts page with two tabs:

• Lists – named groups you build for targeting. A list can contain Users (existing customer accounts) and Contacts.
• Contacts – standalone phone numbers with a name and a Receive SMS? flag, for people who are not customers (leads, walk-ins, imported numbers).

Customers do not need to be added as Contacts; they are already reachable through the Global, subscription, delivery and pickup groups. Use lists when you want a hand-picked audience or want to include non-customers.