It sounds like you're using an analog extension, not a POTS line. Are you dialing 5303 from the intercom path of another system phone? If so, that's the extension number of the fax, but not necessarily the DID. To confirm, from the analog set, call another system extension. The fax ext should appear on the display.
To assign a DID to that set, in installer programming, go to Lines, and program a Target number with the receive digits 5303 (assuming 5303 is in the DID block assigned by your CO provider). Set the Prime set for that line to ext 5303.
Go to Terminals&Sets, 5303, and show until it prompts you for a line number. Enter the target line you programmed in the last step, and change it to Ring Only.
Exit programming, wait for the system to update, and test again.