Problems with Asterisk + Gateway GSM

Hello, I´m using a gsm gateway MT9960 Mobile Trunk. It works fine when I connect it to a PBX (Panasonic) or to a phone. Now I´m trying to connect it to an asterisk. My asterisk has a FXO card (OpenVox), and work fine with a land line.
The problems when I connect my asterisk to the TECOM gateway are these:
1.There are a lot of noise in the line, so the call is hanged up before this finish.
2.When I call someone the call is ringing, and when the other part takes tha call asterisk detect it as a hangup.

I would like you to help me, I think it could be a voltage problem, I tried changing the singalling in asterisk but is tha same
Any clue?

Thanks in advence