FASTRACK M1306B ATH command problem



I have a problem with a M1306B modem.
After an incoming call I give an ATA command, what works well, but if I try to give ATH command (after ATA), the line disconnects, but the modem gives no response. I have to reset it. The firmware is: 643c09gg.Q2406B 1371016 093005 13:2

Has anyone solution or experience with same problem?
Could an firmware upgrade solve my problem? How can I download the firmware?