how to make old telephones as intercom

I want to make 2 telephones work as a intercom. i searched in google and found the technique in doing that
**broken link removed**
This is link for making intercom. But my doubt is, we connected those wires ok. But which number i have to press from one telephone to ring the other telephone and viceversa ?

It's not a very good link (EDIT: actually, it's ok link. It does mention the ringer will not work). This one explains it. It explains that a higher voltage AC signal is used to cause ringing voltage.

But which number i have to press from one telephone to ring the other telephone and viceversa ?

Click to expand...

You can't press a button on a phone to generate this. They don't have this functionality. Phones are one-way; they
can be used to receive a ringing voltage (and therefore generate ringing) but there is no button you could press to
cause ringing. If you really wanted to press a digit button to cause ringing, then you're not really in intercom realm,
you're in PBX or key system realm. There, you'd need to have a circuit that uses a DTMF decoder to listen to the
digit being pressed, and then generate the ringing voltage. It can be done, but is more effort than it is worth just for
an intercom (an intercom can be purchased for cheaper than you could make a PBX circuit), for example, see here.

without pressing button, can i make ring ?
i.e., when i lift first phone, the other phone has to ring...is it possible ?

The link I posted describes how to create a ringing voltage. If you can detect off hook (by the phone presenting a load, see here under 'basic signaling) and then switch on your ringing voltage, then connect the two lines together once the other party is off hook too, then sure.
Before you ask, I don't have a ready-designed circuit to do this, that would be something you'd need to research and design. But the elements of what you would need (ringing voltage, off-hook detector) are clear.

Thank you so much...i'll search for that...