What will you need before you can set up conversion tracking for calls from an ad or from a website (assuming google forwarding numbers are available in your country) ?

Correct answer: An active call extension or call-only ad.

This kind of conversation tracking tracks a call as a conversation when it lasts than a minimum length you set. This way, you can filter out short calls that probably don't include sales or other valuable actions for your business.

If you use another system to track when calls lead to sales or other conversations, you might want to create an import calls conversion action instead.