UDP is a connectionless protocol. I doubt that it is possible for netstat to show a UDP port as connected.
You need to define connected. Do you mean registered? If so, you need the protocol logs from Asterisk. Asterisk has bound to the port, so “pjsip set logger on” should result in it logging the REGISTER request, if it gets as far as Asterisk. If it doesn’t get logged, you need to look at the firewall on the Asterisk machine, then everything upstream.
Being registered is not something that the Linux kernel, and therefore netstat, knows about.
Note that most phones, and presumably LInphone, register as the same user as they authenticate as, so one would expect both the endpoint and the aor to have the same name, which should be the name used in the From header from the device, so I suspect both are wrong.