Could be compatability issues due to compression settings or modem speed.
Go to System > Admin menu > Fax Settings
Try enabling Error Correction Mode (ECM)
If that doesn't resolve it try just lowering the Modem Speed to something like 14.4 Kbps
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.
I am able to send faxes without receiving any error messages, but actually the person im sending the fax, never receives it, i doubled checked the numbers in the address book and they are correct.
When i scan the doc to fax, it scans like normal, and then the dial tone kicks in and the green blinking light goes off then the fax is sent.
I checked the Job status and it shows the fax went through but the receiver never gets it. There was a case were another staff member sent a fax and the other end received it but it was completely blank.