Multiple simultaneous calls asterisk with voipbuster?

I read on the voipbuster forum that it must be possible to place multiple simultaneous calls on 1 account, if your sip device supports it.

I am using asterisk as a sip device, but when I have one call placed another one on same account gives busy-tone.

Should I do something special in my dialplan, or in sip.conf (call-limit is set to 5)?
Is there anyone who has succeeded in setting up asterisk with voipbuster with multiple calls?