I have noticed myself and several of my friends that moved to Primus because of its cheap rates. We decided to keep Primus despite and slightly lower but acceptable voice quality compared to Bell/Rogers. However, one thing everybody notices that they charge you one-two weeks extra amount on first bill (when you move to Primus) and final bill (when you move back to your original.
None of us had any success to talk to their supervisors regarding this issue.. as it seems it is done on purpose. We had to go through our lawyers to get money back. Only one letter from lawyer works.
Just to inform you if you get stung by the same.