Zong introduces a simple code, *310#, which lets you avail many of its services. With this simple code, you can check your account and subscribe/unsubscribe many services and packages. For example, you can check usage details, recharge your account, activate internet bundles, activate SMS/voice bundles, recharge your account and many more services.
It actually is a USSD portal which lets you perform different operations on your Zong SIM. Here are the details of this new code as mentioned on the official website.