i connect to my server from difrent internet connections
not al devices has the option to use vpn so 5060 need to be open to the internet

when i take a look at the log files i see that there are a few hunderd logon attempts from one single ip

i have blacklisted this in my firewall now

is there a option that the server will block the ip after a X number of logon attempts?

One thing to try is fail2ban.