The 19-year-old beat Williams in three sets to make it to her first ever grand slam final.
Having received numerous congratulatory texts and calls, she said she is wondering how to respond to them all, not to mention the phone bill she will incur.
"I thought it was free to receive messages but someone told me otherwise," said Stephens.
"So I'm still trying to figure out what to do, because otherwise my phone bill is going to be crazy and my mom is going to be like 'Oh my god, your phone bill.'
"She's going to be like 'The money you were going to buy yourself something nice with, you're going to pay your phone bill!'"
Stephens will now face Victoria Azarenka in the semi-final.