@Salvo_C 

So the TD-W8960N works as a wifi repeater to boost wifi signal from router 1 to router 2? 

If you just want TD-W8960N to provide internet for router 2, you can configure it as an access point or repeater. 

https://www.tp-link.com/support/faq/417/ 

https://www.tp-link.com/support/faq/264/

For the VOIP function on router 3, I'm afraid that it has nothing to do with the TD-W8960N. 

Good day. 

Thanks Kevin, 
what I really need is to have  any device attached to Router3 working with Internet service provided by the Router1. The TD-W8960N should helpwith this.
This is a required configuration to have a backup internet connection whenever the VDSL line of the router3 will fall down... and also to verify if I can dismiss the fixed line in the next future.
 

@Kevin_Z 
(see also my previous answer)

Of course I cannot use any ethernet cable to connect R1 to R2 and also I am quite sure that R1 is not supporting the WDS option.

Thanks again.
Bye.