Shahzad, your post reminded me of an old project I had engaged years ago, it was a intrusion alarm: the way it functions was when the circuit detects any,
circuit starts dialing a predetermined directory number and announces the prerecorded (digitally) audio to receiver side.

Before the interface you asked for, here are some advices,

- You probably will be in need of detecting the presence of the Dial-Tone (differs from country to country) If the Switch your telephone set is connected provides the DT as soon as you offhook, you may skip off this step.

- Don not forget that, although your side is assumed to be the originator side, you are very likely to get rings. Ring tones are 60-120V modulated signals that your circuit should protect itself from any damage it may couses.

- You should decide on what sort of dialing method you use. DTMF or DP (DialPulse) DTMF is faster, DT is cheaper

- The line empedans varies from system to system depending on your country has been adopted, most systems are 600 ohm. Learn yours.

As to the circuit, the simplest one is to use a 600 ohm (2 watt) resistor that is connected to the line(phone) over a relay. Relay is used to offhook and onhook the line as well as the dial the digits (Dial_Pulse). If this resistor is connected to an audio circuit (with 470nF caps. at both post of the resistance), this also ables you to send audio.

An audio transformer (instead of the resistance) with an empedance of 600ohm at the line side will do better.
But I would recommend you to use easily available modern circuits available in your marketplace.

Have fun.

Rustu