If you answer and don't show up then the first half of the fare is not transferred from the client and this leads to a bad reputation. Same for #2. #3 can be

If you answer and don't show up then the first half of the fare is not transferred from the client and this leads to a bad reputation. Same for #2. #3 can be avoided based on reputation previously gained. Maybe some sort of signup payment with a trusted vetting authority.