Here is the SIP packet sent by zoiper (below). It is causing an error as there is no user called "yes", and so fail2ban kicks in and blocks the IP address.
Thanks in advance
Clive
REGISTER sip:196.22.x.x;transport=UDP SIP/2.0.
Via: SIP/2.0/UDP 192.168.1.102:57607;branch=z9hG4bK-524287-1---298a99c922eb0aca.
Max-Forwards: 70.
Contact: <sip:yes@192.168.1.102:57607;rinstance=0586945545f5571e;transport=UDP>.
To: <sip:yes@196.22.x.x;transport=UDP>.
From: <sip:yes@196.22.x.x;transport=UDP>;tag=fc50b27f.
Call-ID: DMpxmHxiyC0KRkt4M9Mrtw...
CSeq: 5 REGISTER.
Expires: 60.
Allow: INVITE, ACK, CANCEL, BYE, NOTIFY, REFER, MESSAGE, OPTIONS, INFO, SUBSCRIBE.
User-Agent: Zoiper rd82a609.
Allow-Events: presence, kpml, talk.
Content-Length: 0.