Homemade Landline phone intercom system

[Silex93]

Good day community

I am doing a project which requires me to connect two landline phones together and use them to form an intercom system. Currently the design i ma using can be found here. http://wandel.ca/homepage/phone-intercom.html

I have successfully created this circuit (except i used those small 12V piezoelectric buzzers instead of the sonalerts) but what i am seeking to do now is not using the buzzers anymore (kinda annoying sound) but to use the actual ringing mechanism within the phone.
I have been doing some reading around about how much volts it takes to ring a landline phone and i think it's about 48-70vAC at around 20hz?
But from the circuit i currently have, i am not sure how to implement a similar mechanism whereby if one phone is picked up, the other one is rung and vice-versa. Any help is greatly appreciated

[German_EE]

A phone will only ring if the handset is down (the technical term for this is 'on hook'). So, have a button on each phone that places 90V @ 20 Hz on the line and the internal circuit of the phone will do the rest with the ring tone being heard in the earpiece of the calling phone.