USB sticks only support voice if ^CVOICE returns 0. And to enable audio
streaming on the "Application" port (whatever is returned by AT^DDSETEX=?)
we need to send AT^DDSETEX=<port> after starting the call. After that
the serial port will send and accept signed 16-bit 8000hz PCM audio, or
whatever format is returned by ^CVOICE?.
This patch is a rework of the original implementation by:
Dan Williams <dcbw@redhat.com>
c9a56605ca