SpringBoot application to send notification via phone numbers on SMS
PhoneService class
In my opinion, the PhoneService should be converted to an interface exposing the methods.
public interface PhoneService {
   void sendNotificationByPhoneNumbers(String title, String message, List&lt;String&gt; phoneNumbers);
}

Then, the PhoneServiceImpl class can implement the interface PhoneService.
@Slf4j
@Service
public class PhoneServiceImpl implements PhoneService {

   @Value(&quot;${notificationService.url}&quot;)
   String url;

   @Override
   public void sendNotificationByPhoneNumbers(String title, String message, List&lt;String&gt; phoneNumbers) {
      PhoneContext phoneContext = new PhoneContext(new SendPhoneByPhoneNumbers(url));
      phoneContext.notify(title, message, phoneNumbers);
   }
}

Those’s changes will allow you to use inheritance and make the code easier to refactor if, you want to add any new phone services (new implementations). The old implementation was, in my opinion, a bit confusing and did not add any advantages that I can see.
Prefer Constructor-based Dependency Injection / Setter-based Dependency Injection over Field Injection
In my opinion, the Constructor Dependency Injection should be preferred over the Field Injection since it makes the testing easier and more natural (you can pass the mocks directly into the constructor) without using hardcore ways to inject the dependencies.
Also, even in Spring documentation, the Field Injection is not even mentioned as a way to inject and lots of people don’t recommend it.
Never expose the variables directly in classes
In my opinion, it's a bad habit to have public variables in classes, since you don’t control your own data. Try to hide them by adding the private / protected keyword before them and use getters / setters to change the state. This will give you more control over the class own data.