Welcome Caroline! Start by figuring out a vertical that you would like to focus on. The main areas in pay per call tend to be healthcare, insurance, home services, travel, legal services, financial services, etc.
Find a vertical that you're interested in and/or have or one you have some background with. If you have specific experience with insurance for your business, or with older relatives, for example, think about options within those verticals. That gives you an advantage when figuring out pain points , creatives and target audiences for your campaigns.