From what I've seen, it is a phase, but one that takes awhile to pass. We went through it at my house. Here are a few things we tried that more or less worked.
I got my daughter a worry bead bracelet to run through her fingers to try to manage nervous/bored/habitual chewing. (Her fingers were constantly in her mouth, and she had chewed her fingernails down to the quick.) It helped some. Her teacher authorized discreet gum chewing in class, which was very effective. And we cut her hair short enough so that none of it reached her mouth (with buy-in from her - I did not force the haircut).
While she was getting the short haircut, the stylist mentioned to her that fingernails have more germs than a toilet seat. That was more useful than anything I had managed to say up to that point, because of the "ewww" factor, and because it was said by someone other than her mother. :-)
It also helps for the child to be aware of what she's doing. I noticed that my daughter did it at times of stress (math class!) or when her mind was elsewhere, such as when she was listening to books or watching TV. So I started telling her when she was doing it. Not "stop chewing, " but "Hey, did you know you're chewing?" I taught her to give her hands a job (worry beads, holding a stuffy) instead, and this did help some.
Good luck!