I read this tread...and to connect the Ardiuno to the telephone line DIRECTELY is NUT !!! 
Telephone line is compose of two element. DC ans AC. No ring, a -48 V is present. When ringging, a 90 V ac @ 20 Hz is created. Just the site in retrolefty. Here a site to interface the telephone line. Telephone ringing circuits Just look it up.
To know what type a circuit a telephone ringer is, I "reversed" engineered a few phones ( take them apart -- All certified by Industry Canada < --- Goverment Department ) The circuit include : 1 fuse, a capacitor mylar ( NON polorize ) of 470 uF @ 250 V , a power resistor @ 2 Watt, 2 zener diode of 20 V @ 1 watt ( not so sure ) , 1 rectified diode rated @ 200 V, and a Varactor rated @ 250 V and the use of opto-coupler.
I am making a telephone call counter project, and my design follow the guideline to connect to the phone line. You DONT want the Tel company to sue you or the FED after you.
You can use the web site sudgested by retrolefty to test your project.