This sentence is a Type 3 Conditional (Impossible / Unreal Past).
🔹 Structure of Type 3:
If + Past Perfect, would have / could have + V3
If-clause: If I had known his number
→ Past perfect shows the condition did not happen.
Main clause: I would have called him
→ Imaginary result in the past.
So the correct form is would have + past participle (called).
