Thanks for your answers, I'll retry tonight! And I know the intercom is supposed to work because when I was visiting the condo, I used the intercom to talk to the last owner to unlock the main door of the building. I remembered that one time he told me "I'll change to another telephone to unlock the door" at the end of the conversation.
I don't know if he was usine a telephone line or an internet telephone line.
Unlike me (I use me cellular internet instead), he also had internet so he was using a little box called IVI6000 by the company SITTELLE to switch between the intercom and the telephone line and he has an E-MTA. Here is the little box that he letf when he quit the condo :
»www.sittelletech.com/pdf/IVI6000 ··· M_12.pdf
Is there something to try with it when I will try my intercom tonight? It says in the documentation that you don't need to have a telephone line to make it work and if the E-MTA is disconnected, it switch automatically to the intercom side.
Is there something I can check (like the voltage or something) in the wall jacks or in the plug with a telephone symbol to be sure it is still connected to the intercom if it is not working after I test it tonight?