Miscellaneous Km5050 and Fax problem

Hi guys!!! I have a big problem: km 5050 with a fax system send fax correctly but not receive anything!!! Line phone is ok; the previous machine ( falcon 2 km3035 ) works fine on this line phone. I reset the fax system( SIM 600), change the fax cable. Nothing... Previously, this fax system was installed on km3050 and works fine. Any ideas??? Firmware problems????:(