In Pokemon soul silver and heart gold they tried not to give away to many legendaries so they decided to leave a few out including Regigigas. Although you can still get it from various cheat consoles like; Game shark or Action replay. There is one last way to get it, if you have captured him on one of the previous three games and have 2 ds' you can import (trade) him from them.
Hope this answers you question ;)
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Trade with Pokemon Platinum, Pearl, or Diamond.
You cannot catch any of the Regis (Regirock, Regice, Registeel and Regigigas) in Pokémon HeartGold or SoulSilver.
You cannot get Regigigas in Pokémon SoulSilver, you will have to trade it from Diamond, Pearl or Platinum.
it's not. buy diamond pearl or platinum.
No, you can not catch Phanpy in SoulSilver. It is a version exclusive to HeartGold.