Yes, you could use it that way. ut to be able to call from one box to the other, you will need to dial the I.P. address of the other box.
If you get a gatekeeper,software or hardware, you could just dial the phone number asigned to the remote box.
Idea: If you have the type of phones that have few memories that could be programed to a button on the phone, you could program the whole string in a memory.
Something like “to call the office in Italy, press the red button.