Hi
Not sure about the USA codes.
But here is a How to create a dial connection with 3G modem:
http://aps2.toshiba-tro.de/kb0/HTD9802OF0000R01.htm
There is a dial-up phone number option and in most carriers, the string *99****1# or *99# should work!
Ceck also this 3G database:
http://aps2.toshiba-tro.de/wlan/?page=3ghowto