No response from the device

The call_manager received a response to an INVITE after at least 4 retransmissions

It is a mitigated version of the Unreachable Device case and could occur together with it, due to too short NAT sessions on the router. Normally each invite should receive a 100 Trying response within a few milliseconds of sending. If the INVITE or 100 Trying replica is lost due to an error on the network, a replica of the same INVITE will be sent back and so on until a response is obtained or a timeout expires. In the presence of physiological network errors, the probability that the second sending will not receive a response is very low, even less for a third reply.

If you have to resort to at least 4 messages before receiving a response, it is very likely that there are no network errors but that the requests have been discarded due to a closed NAT, reopened fortuitously to send a new registration packet.

Possible causes:

1. We may find ourselves in the presence of a NAT / Firewall configured to generate NAT UDP sessions with a timeout shorter than the duration of the SIP registration sessions. For example, with a 5-second timeout UDP NAT, all terminals / gateways would register without problems, but the nat would remain “punctured” only for 5 seconds every 60 or 120, making it impossible to contact the terminal / gateway in most of cases.

Possible solutions:

1. Check that the duration of the NAT sessions for the UDP protocol is at least 60 or 120 seconds, depending on what is set on the terminals / gateways