You can't... without Action Replay.
I do not know a legit way to get Regice (along with Registeel and Regirock)
The only Regi I know how to get legit is Regigigas, which needs Regirock Regice and Registeel.
If you know a legit way to get Regirock Regice and Registeel, edit this answer.
actually you can. all you have to do is send Pokemon from ruby, or Sapphire to dimond or pearl or platinum. then trade it to soul silver.
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you have to trade from diamond or pearl
You cannot catch any of the Regis (Regirock, Regice, Registeel and Regigigas) in Pokémon HeartGold or SoulSilver.
You catch regice
No, you can not catch Phanpy in SoulSilver. It is a version exclusive to HeartGold.
the only way you can get regice in heart gold is by trading from Pokemon platinum,diamond and pearl