When in doubt, use the source:

# instances (Device|Win32)::SerialPort object and initializes communic
+ations
sub connect {
...
    # Connect on serial (use different mod for win32)
    if( $me->ostype eq 'windoze' ) {
        $me->port( new Win32::SerialPort($me->{'port'}) );
    } else {
        $me->port( new Device::SerialPort($me->{'port'}) );
    }
...
    # Get the modems attention
    # Send multiple reset commands looking for a sensible response.
    # A small number of modems need time to settle down and start resp
+onding to the serial port
...
    my $init_response = $me->send_init_string($init_string) || '';
...
    $me-> log -> write('info', 'Ok connected' );
    $me-> {'CONNECTED'} = 1;
}
[download]

So connect creates a (Win32|Device)::SerialPort for the Device::Modem, and connects to and initializes it.

sub is_active {
...
    # Modem is active if already connected to a line
    if( $self->flag('CARRIER') ) {
        $self->log->write('info', 'carrier is '.$self->flag('CARRIER')
+.', modem is connected, it should be active');
        $lOk = 1;
    } else {
...
        # If DSR signal is on, modem is active
        my %sig = $self->status();
        $lOk = $sig{DSR};
        undef %sig;

        # If we have no success, try to reset
        if( ! $lOk ) {
            $self->log->write('warning', 'modem not responding... tryi
+ng to reset');
            $lOk = $self->reset();
        }
    }
    $self->log->write('info', 'modem reset result = '.$lOk);
    return $lOk;
}
[download]

And is_active checks for a line connection or DSR signal, and performs a reset if no DSR.

connect would succeed and is_active would fail when the modem can be initialized (occurs on the TxD/RxD lines) but the SerialPort does not see a DSR signal (DSR line). This could be caused by an unlikely failure of the modem hardware, or much more easily by a bad or incorrectly wired modem cable.