Dear Fropa,
I need your help i am facing the same problem this time and i have same asterisk 1.6.2.22 version.
I have also no any switch, router, NAT issue this is on the same network and same suddenly no call reach to agent from the queue then same core channels queue etc,stop start asterisk and again start fine but same suddenly calls not landed to agents from the queue and i have same SIP provider i have also SIP trunk and i am sure this is also my SIP Provider issue but i am not able to justify how can i do this and i can say to SIP provider that what setting they need to check or change.
Please help me to solve my problem.