I follow the typical circuit of the MT8870 as
sketched on the datasheet. The circuit is working
good while the telephone powered with dc 12v in series
with a 100 ohm R. But it doesnt work any more when
the circuits is connected to the telephone line.
After that, the circuit doesnt work with the telephone
powered with the dc supply. No component is damaged after checked. No line in the pcb is cut. But If all the components pull off, and put it again in the same
place then the circuit works again with dc supply.
After connected to the telepone line the same problem
comes out.

What component need to be added to the typical circuit
to ensure the circuits is working at any time?

Please some one out there be kind to tell me.
Thank you in advance.