Interfaced a Toshiba DK424 with a Right Fax Server once. It was work but hours later it worked great.
Try bouncing the start to finish application here and maybe we can get it figured out.
What type of switch/PBX?
How are you interfacing? S/L ports?
How many voice ports and how many fax ports?
Are you using DID's? Directed to S/L ports?