How can I store array values (RLMArray) in Realm DB with Objective C?

You need to create realm model for Numbers array:

#import <Realm/Realm.h>

// Define your models for Numbers array
@interface Numbers : RLMObject
@property NSInteger *num;
@property (strong, nonatomic) NSString *name;
@end
RLM_ARRAY_TYPE(Numbers) // define RLMArray< Numbers >


// Define your models for Numbers array
@interface Hotlines : RLMObject
@property (strong, nonatomic) NSString  *id;       
@property (strong, nonatomic) NSString *department_name;
@property (strong, nonatomic) NSString *flag;
@property (strong,nonatomic) RLMArray<Numbers> *numbers;   
@end

// Implementations
@implementation Numbers
@end // none needed

@implementation Hotlines
@end // none needed

For more information refer Realm Objective c

Update :

 RLMRealm *realm = [RLMRealm defaultRealm];
[realm transactionWithBlock:^{
       Hotlines *obj = [[Hotlines alloc] init]; 
       obj.department_name = @"anyString";
        Numbers *number = [[Numbers alloc] init]
        number.num = 1;
      [obj.numbers addObject:number] 
      [realm addObject:obj];
}];

For multiple data :

[realm transactionWithBlock:^{
    Hotlines *obj = [[Hotlines alloc] init];
    obj.department_name = @"anyString";

    for (int i=0; 1< 10; i++) {
        Numbers *number = [[Numbers alloc] init]
        number.num = i;
        number.name = @"XYZ"
        [obj.numbers addObject:number]
    }
    [realm addObject:obj];
}];

Can be done with primitives if you really just want numbers:

@interface Hotlines : RLMObject
@property NSString *id;       
@property NSString *department_name;
@property NSString *flag;
@property RLMArray<NSNumber*><RLMFloat> *numbers;
@end

From the realm docs:

RLMArrays can store primitive values in lieu of Realm objects. In order to do so, constrain a RLMArray with one of the following protocols: RLMBool, RLMInt, RLMFloat, RLMDouble, RLMString, RLMData, or RLMDate

.