You're not obligated to provide specific feedback on the spot.
You can always simply say:
"Thank you for your interest. I will contact you within (a specific timeframe) if you are selected for the next round of interviews."
If you believe they've passed, it is possible you will decide later that they are not among the strongest candidates, and that you'd prefer not to take the time to bring them in for a second round, especially if you receive a large number of applications. You may find it useful to compare notes on all your candidates before making a commitment to bring anyone back. However, if you are certain they are a strong candidate, you can always end the call by scheduling the next round of interviews.
If you are certain they've failed, there is almost never an upside to telling them so. They will inevitably want to change your mind, ask for a reason, or even possibly become emotional. Since your objective is to quickly screen out those who are a poor fit, putting yourself in the position to need to deal with these complications is counter-productive. Again, you can use the above statement. Saying that you'll contact them if... is not lying, it is simply a polite way to reject them indirectly. Specifying a specific timeframe (ie. will contact them within the next 3 days if...) can be helpful in providing the unsuccessful candidate with certainty about the outcome of the interview.