Twilio: java.lang.NoClassDefFoundError: com/twilio/sdk/TwilioRestException – Twillio

I am trying to send a sms using twillio, here the code:

import com.twilio.sdk.TwilioRestClient;
import com.twilio.sdk.TwilioRestException;
import org.apache.http.NameValuePair;
import org.apache.http.auth.Credentials;
import org.apache.http.message.BasicNameValuePair;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

import java.util.ArrayList;
import java.util.List;
import java.util.logging.Logger;

public class SendSms extends HttpServlet {
    private Logger logger = Logger.getLogger(SendSms.class.getName());
    private TwilioRestClient client;
    private Credentials credentials;

@Override
public void service(HttpServletRequest req, HttpServletResponse resp) throws IOException,
        ServletException {
    final String twilioAccountSid = System.getenv("TWILIO_ACCOUNT_SID");
    final String twilioAuthToken = System.getenv("TWILIO_AUTH_TOKEN");
    final String twilioNumber = System.getenv("TWILIO_NUMBER");
    final String toNumber = (String) twilioNumber;
    String message = "prova";
    client = new TwilioRestClient(twilioAccountSid, twilioAuthToken);
    sendMessage(toNumber, message);
}

public void sendMessage(String to, String message) {
    List<NameValuePair> params = getParams(to, message);

    try {
        this.client.getAccount().getMessageFactory().create(params);
    } catch (TwilioRestException exception) {
        exception.printStackTrace();
    }
}

private List<NameValuePair> getParams(String to, String message) {
    List<NameValuePair> params = new ArrayList<>();
    params.add(new BasicNameValuePair("Body", message));
    params.add(new BasicNameValuePair("To", to));
    params.add(new BasicNameValuePair("From", to));

    return params;
}

}

Is just a test so I don’t matter about performarce, code cleaning and so on. Calling the servlet I am getting the following error:

WARNING: Error for /SendSms
java.lang.NoClassDefFoundError: com/twilio/sdk/TwilioRestException

I am using maven, importing the related twillio apis, here the pom dependency:

<dependencies>
    <dependency>
        <groupId>com.twilio.sdk</groupId>
        <artifactId>twilio-java-sdk</artifactId>
        <version>(6.0,6.9)</version>
    </dependency>

Does someone has some suggestions?
Thank you in advance.