Widely available IC for encording and Decoding DTMF data
Holtek Semiconductor have traditionally had a wide range of DTMF encoders and the Holtek HT9170 may be a good option for a decoder while the Holtek HT9200 is an option for the encoder. The datasheets contain examples of connecting to a microcontroller and they are quite easy to use.

Holtek products often don't seem to be carried by a lot of larger suppliers but I just checked and they still seem to be commonly available from smaller suppliers so you shouldn't have much trouble tracking them down.