If you’re setting up a SIP proxy (I don’t know if a “router” makes sense otherwise. SIP is a 2-way protocol which requires a kind of transparency to support location),
I’d think you’d need to read the documentation related to the software you’re using.
If you’re trying to manage QoS the standard Linux packet shaper is tc.
Linux, as ruter for SIP traffic between LOCAL ASTERSIK server and Users in the Internet. ( Asterisk use udp packets )…
I look for instructions how to check quality of work of a router. how purely the channel between the external user SIP and the Asterisk server in a local network works
As I described, I don’t know what you really mean by a “SIP router” because of what the SIP protocol is or does.
Unless your device is bridging, I’m speculating you have to be running a SIP proxy or forward packets to your Asterisk and even if you forward there can be topographical issues depending on how your clients are deployed particularly if you’re supporting the location attribute.
So, if you’re running a SIP Proxy, please verify and read its documentation.
On the other hand, if you’re just forwarding packets to your Asterisk, then monitor the traffic on your external and internal interfaces (There are many apps that can do this).