register VoIP push notifications with ZeroPush: https://zeropush.com/guide/guide-to-pushkit-and-voip

AppDelegate.h

#import <UIKit/UIKit.h>
#import <PushKit/PushKit.h>

@interface AppDelegate : UIResponder <UIApplicationDelegate, PKPushRegistryDelegate>
@property (strong, nonatomic) UIWindow *window;
@end

AppDelegate.m

#import "ZeroPush.h"

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
  PKPushRegistry * voipRegistry = [[PKPushRegistry alloc] initWithQueue: dispatch_get_main_queue()];
  voipRegistry.delegate = self;
  voipRegistry.desiredPushTypes = [NSSet setWithObject:PKPushTypeVoIP];
}

- (void)pushRegistry:(PKPushRegistry *)registry didUpdatePushCredentials:(PKPushCredentials *)credentials forType:(NSString *)type {
  [[ZeroPush shared] registerDeviceToken:credentials.token];
}

-(void)pushRegistry:(PKPushRegistry *)registry didReceiveIncomingPushWithPayload:(PKPushPayload *)payload forType:(NSString *)type
{
    UILocalNotification *notification = [[UILocalNotification alloc] init];
    NSDictionary *data = [payload.dictionaryPayload objectForKey:@"aps"];
    notification.alertBody = [data objectForKey:@"alert"];
    notification.category = [data objectForKey:@"category"];
    [[UIApplication sharedApplication] presentLocalNotificationNow:notification];
}

@end