code: public ModifyCallResponse modifyCall(String uuid, ModifyCallAction action) throws IOException, NexmoClientException {
          return this.modifyCall(new CallModifier(uuid, action));
      }
docs: Modify an ongoing call.
  <p>
  This method modifies an ongoing call, identified by "uuid". Modifications to the call can be one of:
  <ul>
  <li>Terminate the call (hangup)
  <li>Mute a call leg (mute)
  <li>Unmute a call leg (unmute)
  <li>Earmuff a call leg (earmuff)
  <li>Unearmuff a call leg (unearmuff)
  </ul>
  
  @param uuid   The UUID of the call, obtained from the object returned by {@link #createCall(Call)}.
  This value can be obtained with {@link CallEvent#getUuid()}
  @param action One of: "hangup", "mute", "unmute", "earmuff", "unearmuff"
  
  @return A ModifyCallResponse object, representing the response from the Nexmo Voice API.
  
  @throws IOException          if a network error occurred contacting the Nexmo Voice API.
  @throws NexmoClientException if there was a problem with the Nexmo request or response objects.