Pretty much every phone that has bluetooth (so pretty much every phone) has an AT command interface for controlling the phone. Some base commands are universal due to the fact theat they precede the cellular phones (ATDT, ATA, ATZ, etc). But there are many AT commands implmented specifcally to control or query cellular connection specific data.
Are these commands at least somewhat standardized? If so, how? UMTS vs EVDO, Manufacture, etc..
There is a 3GPP standard for AT commands: http://www.3gpp.org/ftp/Specs/html-info/27007.htm
3GPP is the standards body for the GSM/UMTS world (as you probably know).
The original AT commands came from a set to control a Hayes Modem.
From past experience not all phones support all commands and manufacturers may add their own commands also. Nokia themselves say (referring presumably to just their own phones):
"Different phones support different sets of AT commands. This means that a specific phone might not support all AT commands listed below. Currently there is no authoritative list available of supported AT commands by all phones."