Ciao Antonio. I should have shared the outcome of my problem here. Sorry I didn't.
Per Mike's answer and my findings, I realized that the modems connect and this has nothing to do with the radio. So, when the power button is pressed on the radio's head, the RRC Radio turns the TS480 on and it is expected that the radio turns on. If the radio is on, then the head communicates with the radio and normal radio functioning starts. If the radio at the remote side does not turn on, nothing happens.
So, If the radio does not turn on (power supply not connected for instance), what you and I described happens.
In my case I had replaced a TS480S with a TS480HX. I had to make a power cable with two power connectors to energize the radio. It happened that this cable had a contact issue. In fact the 13.8V DC to the radio was intermittent.
So, again, the modens were connecting and the radio was not energized. Of course I had to drive to the remote site to discover that.
Maybe you should make a similar investigation.
Best regards Fred PY2XB