huh huh huh???
"Too my knowledge Kitty has never passed through someone's Force Field. I know Ultimate Kitty can not, because Invisible Woman knocked her out with one and she even stayed Phased. 616 kitty, not 100% sure, but I do not believe so."so that basically tells me that magneto's forcefields are multi phasic.
In Uncanny X-Men #143 she's in the Danger Room and there's a forcefield with a "random energy pattern" and she says that it would take concentration to phase through it. So maybe she could phase through Magneto's forcefield if she had enough time and could concentrate, but doesn't.