Hi Gyegavian,
Thank you for using the Support Forum. Please take a look at the solution for setting up the machine to send and receive faxes. Make sure that you try a slower speed and see if that will help. The WorkCentre 6505 requires an analog phone line. I understand you are using a ATA but that may be the issue. If the problem continues after slowing the speed down please consider contacting your support centre for further assistance.