check for OS added.

Author: aarkalyk

index.js

@@ -92,10 +92,11 @@ export default class RNCallKit {
     }
 
     static checkIfBusy() {
-      if (Platform.OS !== 'ios') return;
-      _RNCallKit.checkIfBusy
+      return Platform.OS === 'ios'
+        ? _RNCallKit.checkIfBusy()
+        : Promise.reject('RNCallKit.checkIfBusy was called from unsupported OS');
     };
-    
+
     /*
     static setHeldCall(uuid, onHold) {
         if (Platform.OS !== 'ios') return;