Hi admac231,
We deliver several monetization solutions to the market. The first one is the post-call display ads. These are full screen ads that appear after a phone call is completed. These ads are very visual and produce a high conversion rate. You can read more here:
SellAring's Mobile Ad Solution- The Post Call Screen | "Talking Telephone Numbers"
We also deliver ring back audio ads which is what you refer to. This ads are played during the ring back tone time (it's not the same as ring tone. Ring-back tone is the ringing a person hears while waiting for a phone call to be answered).
The phone call may not be initiated by the app. However, we take every measurement to assure all parties win. Since your app remains "banner free", people who download your it enjoy a good user experience. We limit the number of ads played to each user- to a max of two ads per day.
Additionally, we have a quick opt-out method available anytime.