Make sure that configuration is correct on Caller side and it is really sending call to right IP (your server IP).

Check if IP is not blocked

It can be done online checking Blocked IPs

Make sure that iptables are not blocking Caller IP on server side. Command to do that:

# iptables -nL | grep -e "DROP" -e "REJECT" | grep 11.22.33.44

Where 11.22.33.44 is Caller's Public IP address.

If IP is blocked by iptables in your server output, will look like this:

DROP       all  --  11.22.33.44        0.0.0.0/0

or

REJECT     all  --  11.22.33.44            0.0.0.0/0           reject-with

If you are not getting such line, IP is not blocked by iptables on your server.

Unblock IP

iptables -nL --line-numbers | grep 11.22.33.44

output will be like:

4    DROP       all  --  11.22.33.44       0.0.0.0/0

notice that first number and then place in following command to delete rule which block IP:

iptables -D INPUT 4

After this command IP will be unblocked.

If you are sure this IP should never be blocked, do following steps:

Open file /etc/fail2ban/jail.conf with you favorite text editor.

Find the line which starts with

ignoreip = 127.0.0.1

add IP which was blocked to end of this line so it would look like that:

ignoreip = 127.0.0.1 11.22.33.44

restart fail2ban service to apply changes:

service fail2ban restart