ATD is the original Hayes “Dial” command
ATDT was added to specify “Tone” (as opposed to “Pulse” or “Loop-Disconnect”) dialling for PSTN lines.
Obviously, this has no meaning to a GSM modem. Some GSM modems accept it as a synonym for ATD.
Since ATD is the most general, I think that would be the better choice (most PSTN modems can be configured to use Tone dialling for ATD anyhow).