The phone interview may be with a recruiter type or may be with a hiring manager (person you're working directly under). This is usually more of a feel you out type discussion to decide if it's worth it to bring you down for a plant interview.
If you do a plant interview, that will be with the hiring manager and some other folks you'd be working with. Depending on the position, you may interview with 2-4 groups. HR makes you do a bull**** "what was a time when you exhibited (insert desirable trait/action)?" Series of questions. So be ready for that.
There's generally a very relaxed atmosphere, so relax and be normal. The biggest things they're looking for are: do you get **** done and will you fit in with the group.
Wear workboots, khakis/slacks, and a long sleeve buttondown to the plant interview. Skip the suit and tie, only the CEO wears those.
They'll decide that day whether they like you, and you should hear back in a day or two.
If you're doing R&D, legal, marketing, or HR, ignore everything I just said.