Till now, the SMS approval for leave has not been authenticated. However, you can send an email through your mobile. Upon returning from leave, you can seek ex-post facto sanction for your sick leave.
During sickness, you can personally contact your boss to inform them about your condition. Hopefully, your boss will agree to your leave request and provide any necessary assistance. For instance, in the case of hospitalization, your boss may assist you in obtaining medical insurance claims and arrange for an assistant to care for you during your hospital stay.