Ask HN: How do I approach designing and building or picking an audio ad server?

I am working on a small project where a user creates playlists and radio stations on a web platform but has the ability to listen to the music via a dial-in toll free number. My target market is developing markets and the music I am availing is local music in these markets. I am trying to keep it free for users, but perhaps create a revenue stream from ads. Ideally, I would be serving short 10sec ads at certain intervals. I would like to use mostly local ads to keep the experience relevant. In so doing, I am having a tough time approaching the ad server itself. Question are: 1) Do I have to build one myself? 2) Are there any ad servers I can customize and be able to serve via a telephone connection? 3) If so, which ones? 4)If not, what are the good design principles and things to remember when building an ad server? 5) Any recommended readings? Any contributions and suggestions would be much appreciated.