When you are sending SMS, you will either be sending an SMS to an entity or directly to a number. Usually you intend to send an SMS to a user entity.
Sending SMS to an Entity
You will need to load in the entity you wish to send to.
// Along the lines of
$entity = MyEntity::load(123);
// Or usually, with a user entity:
$entity = \Drupal\user\Entity\User::load(123);
Create a reference to the entity phone number service. Typically this is done using dependency injection.