code: public void login() throws WorkspaceApiException {
          try {
              ApiSuccessResponse response = this.voiceApi.loginVoice(null);
              throwIfNotOk("voiceLogin", response);
          } catch (ApiException e) {
              throw new WorkspaceApiException("voiceLogin failed", e);
          }
      }
docs: Login the current agent on the voice channel. When you make this request, Workspace uses the parameters you
  provided in `activateChannels()`. For most applications, you don't need to worry about logging in
  the agent on the voice channel because it's handled by the Workspace API when you call `activateChannels()`.
  However, if you `logout()`, you can then use `login()` to login the agent on the voice channel. Note: This
  login/logout flow only applies to the voice channel, not to the agent's session.