Uable to access some classes under `package android.telephony`

I am trying to access classes stored under android.telephony package using reflection but for some classes I am getting ClassNotFoundException.

Can anyone tell me why some classes can be accessed and some are not even though this classes have same Access Modifier

Ex: public class TelephonyManager can be accessed using reflection as shown below

try {
Class<?> manager1 = Class.forName("android.telephony.TelephonyManager");
} catch (ClassNotFoundException e1) {
e1.printStackTrace();
}

public class SubscriptionManager cannot be accessed using reflection as shown below

try {
Class<?> subscriptionManager = Class.forName("android.telephony.SubscriptionManager");
//Throwing error
} catch (ClassNotFoundException e1) {
e1.printStackTrace(); 
}

both classes have same package android.telephony