I'm guessing you are talking about the episode The One With The Fertility Test (S9Ep21):
Rachel receives a gift certificate to get a free massage at a chain massage center, which Phoebe doesn't want her to take advantage of as she feels it takes away business from independent massage therapists like herself.
What Phoebe doesn't tell Rachel is that she's one of the employees at the very same massage center. However, she has a hard time concealing her identity to Rachel, her client, during her massage.
While Phoebe hates these massage chains she works for one
... because it's good money ... and they give me benefits, like medical and dental...
but she admits about feeling ashamed
... because I sold out for the cash