Hereās a few direct ways:
Setup a Call Only campaign for your ads. This will show your ads to mobile or call enabled devices and direct ALL clicks to immediately call your business.
Or setup a hybrid, where you have the option to show your ads on both desktop and mobile, but set your Call Extensions for that campaign to be call only, where mobile clicks are always directed to call, rather than visit your website.
My preference, has never been to use the Call Only feature whether as a Campaign or an extension. I notice that as a user, I am annoyed that I am forced to call, and am not given the choice to visit or call. So instead, for most of our accounts where we want to encourage calling, we use CTA phrases to encourage calling. e.g. Call for Free Offer! Call Today! Call Now for 20% Off. The overall ad should influence the behavior to call over click to website.
In some cases, if the historic data support its, break off specific keywords that have generated calls, into their own Call Only campaign to maximize the conversions from those keywords.
Secondarily, these are performance improvements to increase inbound calls:
Are your ads showing with the phone number as often as possible? Analyze the data and make changes to increase the frequency of your call extension showing with your ad. This may involve increasing your bids, regrouping your keywords to meet the minimum click threshold, or rewriting ad copy to increase your CTR.
Make sure your website has an easy to click and accessible phone number or call button. Might sound obvious, but many websites fail to do this well. Once I click through to your website, here is another opportunity to encourage your prospects to call you, and your website should make it extremely easy for me to see it and click on it from a mobile phone.