Zoiper is verry prone as of tripping the intrusion detection of Asterisk freepbx. So just check admin - system admin - intrusion detection for your IP. It most certainly will be there in the banned ip list.
Unfortunately freepbx Asterisk is updating its known host list only every 10-15 seconds and zoiper is hitting freepbx with requests right after successful login. so after the required numbers of requests sent from zoiper to freepbx while the zoiper client is not jet in the white list from freepbx, fail2ban puts the zoiper client into jail also the client did not do anything wrong, except for being a bit rushed with talking to the host.
I modified my fail2ban settings to not jail my VPN internal ip anymore.
Not happy with this, but working.
As far As I know the freepbx team knows about this and will change the timeframe for the intrusion detection some day in one of the further updates.
Also zoiper could just stop to behave so badly against the host. No need to hit the host with 50 request inside the first second(s)