Detect inband DTMF in OSS channel


I have just implemented a GSM gateway for my Asterisk box by connecting a GSM modem to the PC soundcard. Everything is OK except that the OSS channel cannot recognize the DTMF tone sent from the remote mobile phone (by hearing from the called party I am sure that the DTMF tones do appear in the audio channel). Thus, I cannot let the remote mobile user to “choose” which extension he/she’s gonna call to.

Are there some workarounds for this problem? Thanks.