Before you can expect any data output this must occur:
This pin presents the output of the demodulator whenever CDET pin is low.
This data stream includes the alternate 1 and 0 pattern, the marking, and
the data. At all other times, this pin is held high.
So, check CDET to be low, which means the carrier was detected. With no carrier(tone), there is no data.
You still have not told us this is connected to a regular telephone line!!!
Paul