Put all your strings in an array, get a random number between 0 and size of array (minus 1), and use that to retrieve the msg. Put in method to make it easy. Define the msg array outside though, for better code.
Something similar to this:
-(NSString*) getRandomMsg {
NSArray *arr = @[@"Give me a sec", @"Hold on"];
int randNum = arc4random_uniform([arr count]); // or arc4random, possibly rand % max
NSString *msg = arr[randNum];
return msg;
}