My bet is that self-server option has a higher per-transation yield, but the over-the-phone option has a higher closing rate. In fact, I worked with a company

My bet is that self-server option has a higher per-transation yield, but the over-the-phone option has a higher closing rate. In fact, I worked with a company that had both options implemented, and pretty much all inbound sales calls resulted in sales. If people are calling, then they are usually ready to buy - from you or from someone else. A good sales person will make sure it's you.