i hv had worked on this issue , which way u wana detect ring tone... by micro or discret component ?...

its hving normaly 440Hz with a pause about2/3 second..... u can detect this frequency by LM567 tone decoder or now a days there are many ics for this perpose,
if u hv plan with micro u hv to write some code for this Mark & space + tone

good luck

hv fun :)