You'll want to store the callback ID as a property on the class.
@interface Sms
@property (nonatomic, strong) NSString *callbackId
@end
And then store it when you are in your send method.
- (void)send:(CDVInvokedUrlCommand*)command {
self.callbackId = command.callbackId;
You can then access it again from your delegate method:
NSString *callbackId = self.callbackId;
You should be good to go.