Had the same problem as Cíntia Pinheiro and others i.e. got “Invalid or no response from the device”. My modem was in NoEcho and NotVerbose mode (ate0v0). Therefore it answered the initializing AT command with “0″ (meaning OK). AT Command Tester does not recognize “0″ as a valid answer.
Using HyperTerm to send ATE1V1 (echo ON, verbose ON) fixed the problem.