If you only used the phone for incoming and outgoing calls then obviously there is some mistake by PCCW. However, I have never heard of a case where PCCW's billing system did actually make such a mistake. So I conclude that it is most likely that you did actually use the phone for some services apart from voice calls. And if you did use it then you have to pay.
Are you really saying that you did not use the phone for any services except incoming and outgoing voice calls?