[bbob]

Strange I have voicetrading running on my frtizbox at home and also on pbxes. I can use both at the same time and both have different ip.

You don't want to authenticate your ip address not at pbxes because this is an ipaddress used by many other users.

See this info from their site:

I can't connect to your SIP server by using username & Password. Can I connect by using fixed IP addresses?
Yes, you can authenticate your fixed IP addresses by choosing 'IP config' in the myaccount area.

Important! If you are using a hosted VoiP softswitch provider like eq. Kayote Networks (etc), make sure you are ONLY using IP addresses which are uniquely assigned to your account (check this with you provider). This is to prevent that other users of the same VoiP softswitch provider are able to use your balance.

so only when you can't use username and password you need to autenticate your ip. But as you see when you do that you need to be the only one on that ip otherwise others can also use your account.

How many simultaneous calls can I setup?
Voicetrading does not limit the number of simultaneous calls. The only limit is the bandwidth of your local internet link.