Hi Andrewtdam,
Thank you for using the Support Forum. Please plug a regular phone in to the phone line being used for the fax machine. See if you can call the fax number you are trying to use. You should hear the tones from the other fax. If this does not work there may be an issue with the number you are dialing or with the phone line itself. If you do hear the fax tones then connect the phone line back to the Xerox WC 6505 and make sure that you are dialing the number exactly as you did on the phone. It is still fails try another fax number, if it is failing for all numbers but the phone works then please consider contacting your support centre for further assistance.