7050 and voipuster-->error(503) with sipdiscount--->OK

[zooster]

Why when I try with voipbuster I get this

Internettelefonie mit ############ über sip1.voipbuster.com fehlgeschlagen. Fehlergrund: (503)

And it's ok with sipdiscount?
I've tried with connectionserver.voipbuster.com, sip.voipbuster.com and sip1.voipuster.com
Where is the matter?

 

[zooster]

Doesn't anyone have this problem??