Pretty nebulous question, depends on what type of interface you are interested in. Spi, i2c, pulse or dtmf tone dialing, terminal emulation, home automation interface, modem, caller id, etc., etc.
Ah, hah! The standard dtmf receiver interface chip is the MT8870. Google that for all the info you need. I found that the MT8888C by Zarlink incorporates a dtmf transmitter and an intel micro interface (including the 8051/2) onboard the IC. Under $3, too.
Here's a link for the Zarlink MT8888C data sheet- **broken link removed**
And the application note for interfacing to common micros- **broken link removed**
You might be very interested in the section "Remote controlling using telephone" in this link.
If those more experienced than me with this type of circuit don't jump into this thread, you probably will find all the help you need in any of the 8051 micro forums.
i want a circuit idea about how i can interface telephone line to 89c51....
i actually requires a caller id. and which in turn connected to lcd display and to pc to monitor the calls..... the circuit also have an autoteller facility..
i connected 8051 with mt8870.
if i dialled no then am i abel to detect indication about dialled no line whether the person hook up the phone, or phone busy etc.