I am using a GSM gateway (a cheap FM Cellroute) connected to a Digium TDM400 FXO port and it generally works pretty well. I have recently be trying to set it up to allow DISA access when receiving calls. I can call the GSM Gateway and it picks up fine.
The problem comes when I try to get Asterisk to read DTMF tones (I am putting a security PIN onto the DISA). When I type numbers of the phone calling the gateway the DTMF tones are not read consistently. Numbers tend to get duplicated but not in any consistent way. For example if I type 5678 Asterisk will read 55677788888 or something similar (but not always the same). I don’t know if this is some sort of echo problem but there is no echo when using the gateway for voice calls.