Hello Dario.
The best test will be getting a ATA 186 or similar, put it next to the E3000, on the same network and make same calls, going VOIP only inside your network,trought the Quintum and back out PSTN.
That will tell you if the gateway is correct or not.
After that same configuration but going trought the internet, someone distant.
I could do that for you if you need it, make some calls from the outside.
The faster that you get this trouble resolved, the earlier that you could be in Gorlero, looking at Tangas, o taking a dip in La Mansa o La Brava.
Since I am freezing my rear end up north, at least I could help someone on the right place.
Write me to Mike3799@yahoo.com