More than likely, Phyllis was probably just blamed for all the stuff that Ang was doing, and then when Ang caught wind of people suspecting her, she just piled on all the trappings, like she did with Vicki. I mean, by the standards of witch-trial logic, you really didn't need much to convict someone.
They wouldn't have suspected Ang outright because not only was she so nice and sweet to everyone's face, but she also came with the DuPres family, who were prestigious people, not to mention people that were practically family, so to accuse one of their servants would've been a huge insult, and vice versa for the DuPres to accuse a Collins servant. Phyllis was the newest employee, and she was also the most unattached to the family, so I'm sure they had no problem throwing her under the bus.