In order to receive Webhooks, you should have the following IPs from billwerk whitelisted:
Production system:
18.185.235.200
18.185.72.97
18.185.233.230
18.195.227.12
3.72.179.65
3.126.156.214
Sandbox system:
3.122.11.92 35.159.47.238
18.196.237.249
3.66.238.169
35.156.177.245
35.157.4.116