You do not need the field group key when updating a repeater
$repeater_key = 'field_6050d513a3996';
$value = array(
array(
'field_6050d550a3997' => '(123)456-7890',
'field_6050d596a3998' => 'phone number description'
)
);
update_field($repeater_key, $value, $post_id);