Sorry for very late.
The ERICSSON link is not working for me.
Check the supplier about the FAX Models you are trying to use.
I have heard the same problem with other guys.They found it was due to some SW misamatch in that FIXED Terminals.
At the end they gone for PC Based FAX Applications after getting tired with these products.
But they told me that there is a solution by UPGRADING/Changing the SW.