asterisk users - Sep 2016 - SIP trunk down. Wireshark shows ICMP Communication administratively filtered

Hello,

I've got a remote system that is plagued with a strange issue.

It happens from time to time.
Yet, I've not found any condition that trigger this phenomenon.

Here is my setup:

-   PSTN  <---> ITSP <--SIP trunk--> Router <----> Switch
<----> Asterisk
box

|

|
                                                                        SIP
Phones

- Asterisk box is a Raspi/Rasbian with Asterisk 1.8.13
- Switch is DLink DGS-1210-10P
- Router is Zyxel box (I don't manage it)


I see (with Asterisk console) that Asterisk is sending OPTIONS/REGISTER
requests.

With tcpdump and wireshark, I see that :
1. those OPTIONS/REGISTER messages are sent to my ITSP gateway (IP src is
Asterisk box, IP dst is ITSP gateway)
2. my router replies (within 30ms)  with something Wireshark presents as an
ICMP message like this:
 ICMP    491    Destination unreachable (Communication administratively
filtered) ( (IP src is router box, IP dst is Asterisk box)


While this happens, I can successively ping my ITSP gateway.

Searching the web, I found [1] and [2].

1. Is it common to get an ICMP reply to a SIP message ?
2. Any advice or suggestion ?


[1]
https://ask.wireshark.org/questions/26667/have-packet-capture-somethings-wrong
[2] http://www.tahi.org/sip-ipv6/ua6/doc-1.0/sip-ipv6-tm/icmp/UA-15-2-1.html


Best regards
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