Incoming Call 503 Service Unavailable


I’m connected to SIP Server and want to phone via Snom Softphone, which is connected to asterisk@home.

Outgoing calls work perfektly, but I can’t receive incoming calls. I’m using ethereal to sniff the traffic of the asterisk box.
When I try to phone my SIP Account, there is firstly an SIP Invite packet from my SIP Provider to my Asterisk box. Then the Asterisk box answers with Status: 100 Trying and furthermore send a packet Status: 503 Service Unavailable.

Has somebody an idea, how to solve the problem? Where can I get more information?

Thanks in advance