Add a call extension

Creates a call asset — a phone number shown next to the ad — and links it to the account, a campaign or an ad group. Use when: the user gave you the number and the country it belongs to, and someone actually answers it during the hours the ads run. Do not use when: the number is not staffed, or the user has not confirmed it. Never guess a phone number. The number must be in the country given by country_code and pass Google's verification; numbers that fail verification stop serving silently. Calls are charged like clicks. An identical number already linked at the same level is skipped. If you are unsure which account, campaign or period the user means, ask them — do not guess ids or dates.