Some formats of online advertisements permit users to call the advertiser directly by clicking on the ad. Such ads can result in customer dissatisfaction when the advertiser is busy or unavailable to answer calls placed through the ad. The techniques described herein employ selective activation of call ads to improve the likelihood that calls made by customers to advertisers are answered. Contextual factors such as location, time, activity that an advertiser is engaged in, etc. for which an advertiser provides consent are utilized to predict advertiser availability. The techniques improve customer satisfaction and can reduce wasteful ad expenditure.