The best you can do is pass the phone number as a NSString. Why convert it to a number? it can even contain a + sign. You can trim all unwanted characters by doing:
NSCharacterSet *charactersToRemove = [[NSCharacterSet characterSetWithCharactersInString:@"+0123456789"] invertedSet];
NSString *newPhoneNumber = [[self.phoneTextField.text componentsSeparatedByCharactersInSet:charactersToRemove] componentsJoinedByString:@""];
newPhoneNumber = [NSString stringWithFormat:@"tel:%@", newPhoneNumber];
[[UIApplication sharedApplication] openURL: [NSURL URLWithString:newPhoneNumber]];