Anyway I did at a fast cat /proc/net/nf_conntrack | grep 5060 to get all connection tracking entries for SIP. And I found more than one, here is on example.

via nf_conntrack and the conntrack program | Robert Penz Blog.

More info on conntrack here…

The timeout for this entry is 180 sec and 172 seconds to go, and the SIP client was all the time sending new probes and therefore the connection was never dropped. What can you do in this instance? You can install conntrack. It is a userspace command line program targeted at system administrators. It enables you to view and manage the in-kernel connection tracking state table. If you want to take a look at the manual without installing it (apt-get install conntrack) you can take a look at this webpage which contains the man page. With this program I did delete the entries with the wrong IP address and everything worked again.