Hi
Not sure if this is even support on VoIP, VoIP was never desined to handle data communications, only voice. said that fax was supported after the development of the T.38 codec so this might be a future supported application, but at the moment, i very much doubt that you will find a VoIP provider to support the process of processing credit card payments etc.
The best opton would be to find a credit card processing provider that provides a terminal that can connect to your network via RJ45 connection and process the transcations via a internet connection.