If you're using IP connectivity then there is plenty of bandwidth. The Speex codec (speex.org) can get down below 5000 bits/sec and sound ok, or 2 kbit if you don't mind robotic sound. If you drop some frames here and there due to errors, it is ok. I don't know what it takes to get data through a GSM voice channel but the standard GSM codec uses 13 kilobits/sec, so if you can bypass the codec and get a bit stream into that pipe somehow, you are fine. If not, it is probably hopeless due to the way the GSM codec would mess up any signal you put into it. I know there are some very low bandwidth speech codecs that get below 1 kbit/sec but sound terrible. Even that may be pushing it though.