Search the technical support sites of Modem vendors (3-Com/US Robotics, etc) or chipset manufacturers (Lucent, etc).

You will find that the "AT" command set is anything but "standard!" (because the "AT" commands were originally proprietary/copyright/trademarked/etc property of Hayes).

I seem to remember hearing that they've now beed codified into formal ISO/ANSI/ITU standards - can anyone confirm or dispel this rumour?