Zebra TC 20 - IMEI not found

Hi, I want to get serial number from TC 20?

public static String getIMEI(Context context) {
   TelephonyManager tm = (TelephonyManager) context.getSystemService(Context.TELEPHONY_SERVICE);
   return tm.getDeviceId();
}

this code return null for Zebra Tc 20. Could anyone give me information, how can I get imei/serial number from tc 20?