How to check user entered phone number is valid or not in Objective c?

Try this one:---

NSString* yourPhoneNo=@"44557788991";

NSString *regex = @"^((\\+)|(00))[0-9]{6,14}$";
NSPredicate *validation = [NSPredicate predicateWithFormat:@"SELF MATCHES %@", regex];

BOOL isValidate = [validation evaluateWithObject:yourPhoneNo];

Or this one:---

NSString *stringToBeTested = @"8123456789";

NSString *mobileNumberPattern = @"[789][0-9]{9}";
NSPredicate *mobileNumberPred = [NSPredicate predicateWithFormat:@"SELF MATCHES %@", mobileNumberPattern];

BOOL matched = [mobileNumberPred evaluateWithObject:stringToBeTested];

I think you want to check the number country wise. for this you have to use following demo

https://github.com/rmaddy/RMPhoneFormat

for UK

    RMPhoneFormat *fmt = [[RMPhoneFormat alloc] initWithDefaultCountry:@"uk"];
    NSString *numberString = // the phone number to format
    NSString *formattedNumber = [fmt format:numberString];

for Australia

RMPhoneFormat *fmt = [RMPhoneFormat instance];
NSString *callingCode = [fmt callingCodeForCountryCode:@"AU"]; // Australia - returns 61
NSString *defaultCallingCode = [fmt defaultCallingCode]; // based on current Region Format (locale)

and so on...