My employer refuses to give me my paycheck unless I come in off the clock for a meeting. He says he has a couple of questions. I've tried to talk to him on the phone, but he refuses.
To answer your question specifically, No, your employer cannot withhold your paycheck indefinitely, or even for a short time. I am not a lawyer, though, so you can double-check with one if you want.
There are at least couple of us, though, that have seen situations like this before, and feel that this is likely your final paycheck, and your boss is offering you the courtesy of being able to explain yourself (and maybe save your job), or at least to hear about his decision in person.
It went down like that for me as a teenager working at a BBQ restaurant. I had wondered out loud on a busy Friday night how much money my boss made on a night like that. On Monday, he called me to come in and pick up my check and that he wanted to talk with me. I got a (too long) lecture about how hard it is to run a restaurant profitably, how I needed a better attitude, and then I was dismissed.
This is most likely your final paycheck.
Edit 01: I am assuming that you are in the United States. This said, each state has their individual laws regarding 'last paychecks'. Some states (such as CA) requires that the check is given immediately (if fired) or within 72hrs (if you quit). Other might not have anything.
If you are in another country, different rules apply.
Without knowing your location, this is impossible to know.
This said, and although not an explicit question by you. It would seem that you would like to know why you are being asked to meet with your boss off the clock for your final paycheck.
Again, depending on your employer and state laws (I am going to use CA as the example), the actions surrounding your final paycheck will vary. Since paychecks in CA are immediate upon termination, your final 'hour' worked will literally be that last meeting. Hence, the no paycheck unless you meet face to face.
It sucks, but then again such is many things in life. Try not to cause a commotion. Keep your chin up and walk out of there with dignity. File for unemployment if you qualify and search for a new job.