
Dial Up modem's days are long gone but Land line still exists either in the form of home pbx or PSTN, An Arduino based Dial Up modem shield can be used for a lot of fun projects.
I'm no electronics expert but modem chips are already available with AT Command support
http://www.conexant.com/Product/modems/embmodems/CX93040/Pages/default.aspx
Reference Modem Design
https://www.scribd.com/doc/293690360/Conexant-Embedded-Modem-reference-design
I think Arduino can produce this shield in no time what do you think? :~
It can be used as
1-Cold Caller Blocker
2-Telephone recording with Caller ID
3-Fax receiving/saving
4-Answering machine
5-Interactive voice response system
6-Home automation over landline
7-Foundation for Arduino based PABX
8-DATA Communication
9-Auto dialer (Connected over blutooth to smart phone's phone directory)
10- GSM to Landline gateway
11- Smart Telephone set
12- Call forwarding