It's possible Call Only Ads are not designed for businesses that don't NEED a call. Urgency is key. If you can find urgency and higher intent KWs for your Niche, then by all means, go for it. If not, start looking at verticals where urgency is almost implied or buyer intent is much higher and in service/product is in greater demand.
Our software drop will be able to drive calls for you through regular web traffic (paid or SEO) and generate a phone call. Regular web traffic is more suited to certain verticals: mortgage leads and granite counter tops as examples.