Hello,
Appreciate you comments regarding this problem:
a) We have a few softphones registered to an Asterisk server.
b) After working fine during a few hours, suddlenty, all the softphones, at once, stop listening to their counterparts. 
c) On the wireshark log, on each softphone PC, we see a āunreachable RTP port destinationā ICMP message, being the SERVER such destination.
Appreciate your thoughts, on what could be the reasons for the server to drop all the RTP ports at once.
The sofphones were still registered to the Asterisk server.
Thanks a lot
Saverio