I will call you upon confirming if it is in stock.
I will give you a call as soon as I confirm if it is in stock.
"在庫の確認ができ次第，電話で連絡します"
電話連絡する call, ring, give a call, telephoneなど
～次第 upon ~ing, as soon as ~
か在庫があるかどうか if it is in stock
確認する confirm
I will call you upon confirming if it is in stock.
I will give you a call as soon as I confirm if it is in stock.
日本 
