If you are using a TDK, SiLabs, Cermetek or Xecom modems; the 1st thing i would advise is to "RESET" the device. I do this every time i establish a new "session" with the modem"

This places the modem in a known state electrically and "logically". Also be advised that just after reset you will often get "bad" characters which you should ignore.

I use these modem types quite a bit and i follow the following general steps.

1) RESET the modem (electrical) not ATZ
2) Wait a period of time for the modem to initialize itself internally. (depends on modem but 50ms. has been enough to date).
3) Flush the rx buffer i.e. Read and dump waiting characters or set In/Out pointers equal (you are using interrupt rx buffering aren't you?)
3)Send an AT command and wait for modem response to determine if the modem is available. The command i use is

ATV0E0,cr,lf

which is to say "don't echo and send numeric responses" the response being:

cr,0

A simple ---> AT,cr,lf

would suffice as you may want the lexical response "OK", "CONNECT", etc. I find the numeric responses and the lack of echoing easier to deal with.

regards,
p