how to convert the on-hook to off-hook automatically? i need to controll the circuit through telephone.

 

What are you tring to do and how? This is not clear.

Are you tring to put a servo or selonoid on the handset?

 

What do you want to do? If you want to ring the phone and have it go off hook and do something, you just have to sense the ring tone and put a load on the line to simulate "off hook". I think I have a circuit somewhere.

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just use a relay to put a 1K/1W resistor across the line to make it off-hook.

 

i need to control the circuit i.e when i call a telephone no. it will rings then if it goes to off-hook after two rings then i will press the no.1 button in the telephone, that will receive at receiver side by using the DTMF receiver then based on the output i will switch the light or fan using relay

 

u need a ring detector ckt (like http://www.tkk.fi/Misc/Electronics/c...ne_ringer.html ) , and count the nof rings (4017), after(2) rings enable a relay such that ur dtmf decoder(8870) is connected to the telephone line .
also the relay should engage a <1K resistor between the telephone lines to simulate an off-hook condition.