WIndows Softphone question

I have an Asterisk Server running on an internal network with Windows and Mac Clients.
The Mac client is running Xlite and can connect without a problem to the server.
The windows client has Xlite, SJ Phone, Express Talk and PortSIP and none of them will sucessfully connect to the asterisk server.
The closest I get is with SJPhone which will register with the server and even recieve calls from the Mac client (which work fine), but as soon as I try to dial out it fails and from then on I can’t even connect from the MAC client, it rings and then goes to voicemail.
Also, if I leave SJphone alone, it will eventually attempt to re-register which fails.

Any Ideas?

Many Thanks