When you first purchase your StarHub Happy Prepaid SIM card, you will find the associated phone number printed on sticker strips pasted on the packaging. The sticker strips look like this:
In this case, the associlated mobile number is 82550791.
If you have accidentally thrown the packaging with the sticker away, you can insert the SIM card into your phone and dial
Your number, balance and expiry date will be displayed on your screen.
Hope this helps!