Connecting an Arduino to a telephone line?
You can interface arduino to telephone line using MT8870D chip. This IC can decode DTMF tones. Below is a reference schematic from the datasheet.



Another popular design floating on the internet is below:



There are pre-build modules available from futurelec :



You can use Holtek HT9200 IC to encode DTMF that can be interfaced with Arduino. Below is the reference schematic from its datasheet.