To have a "few" non-performing team members means that you're doing something wrong yourself, maybe the way you're managing the project is wrong. Are you giving vague requirements? Are you micromanaging? Are you reprimanding your team members in public? Are you playing favorite? These four things that some project managers do can frustrate team member, and can cause this low productivity that you're talking about.
So before trying to tackle the issue, try to see if you are the problem (which is true if you can answer yes to any of the above).
Now, if you're not the problem, then all you need to do is to talk to the team member, in your office, don't call him using the phone, just go to his desk, and ask him if you can meet with him in your office in a few minutes.
Now, you have to be very careful at this point, you have to reassure the person that what you're doing has nothing to do with your ego, and you are there to discuss the performance for the better of the team member, the team, the project, and ultimately the company.
You should start your questions with, "is there any personal problem that you're having?". I've seen several occasions where team members break into tears at this point, because some of them really have personal issues that is affecting their work. If he says yes, then all you can tell him is "if you need a few days off then I will try to get them for you". Speaking about the performance at this point is useless.
Now if the person replies he has no personal issues, then you can ask him, very gently, "What's the problem? What's happening to you? Why is your work productivity lower than the usual amount?". "Are you mad at anyone in the company, are you mad with me?" At this point you might get a real answer (which will tell you what you need to do to solve the problem), or just silence. If you're getting silence, then you can push a bit more, like "We can't continue like this... I want you to be happy, and I need to know what's bothering you and what's affecting your performance, please let me know what I should do to make your performance back to the team standards." At one point, he will reply with what's bothering him, you will make a deal with him to fix the issues (it might be a conflict with another team member), and he promises you he'll get better.
Now if he DOESN'T get better, then at this point you need to contact his direct manager and make him aware of this problem. His manager usually has much more authority than you over him, and he knows how to resolve this issue (resolution can mean firing the team member, so be warned).