Hello Mr. Talhaz Hakim, 

Searching www.google.com I came across this web site
(http://wandel.ca/homepage/pbx.html). It describes how to build a
Homemade PBX. It uses a 8031 microprocessor and has lots of
schmatics which show you how to hook up telephone extensions.
Check it out.
I hope this is helpful. 

Good luck. 

Regards, 

Charles Bannister



Specifications 
Eight telephone extensions with roughly telco spec voltages and currents
(48V onhook, 90VRMS 20Hz sinusoidal ringing, about 25mA loop current
offhook.) Ring trip is sub-spec but can handle at least 3 "500" type
rotary dial telephone sets in parallel without false tripping. Lines
are not balanced, nor is one side ground. 

One central office line capable of inbound calls (ring detector) and
outbound calls with DTMF and pulse dialing (selectable, independent of
the type of extension phones. That is, the PBX can convert tone-pulse
and pulse-tone.) 

Three internal voice buses, meaning up to three calls in progress
simultaneously. 

Telco standard call progress tones (dial, busy, fast busy, audible ringing.)