Unless you have error detection / correction, etc, I don't think the AT would add anything else to the equation :-?
Now what do you do? Well, you check the first byte and see if it's a valid first byte for a command. There are hundreds of them, so basically every uppercase ASCII character should do.
In the happy path, you get a byte stream like "DT 1234\r\n".
Sometimes, the data stream has noise in it, so you get bytes like "%$DT 1234\r\n".
There are no commands that start with "%" or "$" so you can just drop it.
However, in the unhappy path, you might get bytes like "CMDT 1234\r\n".
CM is a valid command! But "DT 1234" are not valid arguments.
If only there was a way to get the ATtention of the parser before parsing commands...
(More or less; there is the obvious ambiguity of something on the remote end also responding to AT with OK.)
This useful AT just doesn't have to be a prefix of every other command.
AT
OK # I have the attention of the modem)
DT5553535 # dial this number now that I have your attention)
DIALING
CONNECT