Hi Suraj,
you expect to receive audio tones in the frequency range 0.3 to 3.4 KHz- from the telephone line. as per your diagram you have coupled it using 100pF capacitors on each limb. and there after you have adopted two nos of 56K on each limb.
if you analyse the impedance of this for audio frequencies - you will appreciate that thhe circuit is under coupled. generallt we beleive that telephone impedance is 600 ohms. so anything above 10*600= 6K ohms is amounting high impedance. and the capacitors need to be around 1uF
more audiowill come to the 8870 and there is no need for maiking the circuit more sensitive.--
only you have to have guarding say two diodes in opposite parallel across the 8870 input, instead of 5.6 ZPYs after the isolation caps. this is beacuse, the audio level will generally be around -6 to -10 dbm. this will work out to less than few mV
hope this should serve you
Sarma