Optimize for the native schedule event. It is facebook's standard book-a-call event and it will almost always outperform a custom conversion because of the history behind it.
Here is why that history matters. When you tell facebook you want a schedule, it already knows what a person who books a call looks like across millions of accounts. It says okay, I will go find you the person most likely to book. When you tell facebook you want a custom event it has never optimized for before, it does not really know what you are asking for. You are starting its education from zero.
If you want the data from your custom event, keep firing it. There is nothing wrong with having both events fire on the same page. But the event you optimize for should be schedule.
One more thing. Keep your campaigns clean. One event per campaign. If you are testing a custom event against the native schedule event, run them as two separate campaigns so each one has a single optimization point. It makes the data easier to read and it keeps facebook focused.